Warning C4267 in vector_indexing_suite.hpp and indexing_suite_detail.hpp

Usage of boost::python::vector_indexing_suite leads to a warning :
`boost/python/suite/indexing/detail/indexing_suite_detail.hpp(619): warning C4267: '+=': conversion from 'size_t' to 'long', possible loss of data`

Here's a snippet of code that produces the warning.
```
#pragma warning(push)
#pragma warning(disable:4244)
// See https://svn.boost.org/trac10/ticket/3353 for 4244
#include
#pragma warning(pop)

void test()
{
boost::python::class_>("DoubleVec")
.def(boost::python::vector_indexing_suite >())
;
}
```

Here's the output of VS2017:
```
1>test.cpp
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/vector_indexing_suite.hpp(176): warning C4267: '+=': conversion from 'size_t' to 'long', possible loss of data
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/vector_indexing_suite.hpp(170): note: while compiling class template member function 'unsigned __int64 boost::python::vector_indexing_suite>,false,boost::python::detail::final_vector_derived_policies>::convert_index(Container &,PyObject *)'
1> with
1> [
1> _Ty=double,
1> Container=std::vector>
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/indexing_suite.hpp(261): note: see reference to function template instantiation 'unsigned __int64 boost::python::vector_indexing_suite>,false,boost::python::detail::final_vector_derived_policies>::convert_index(Container &,PyObject *)' being compiled
1> with
1> [
1> _Ty=double,
1> Container=std::vector>
1> ]
1>C:\Users\Cyril\dev\test.cpp(9): note: see reference to class template instantiation 'boost::python::vector_indexing_suite>,false,boost::python::detail::final_vector_derived_policies>' being compiled
1> with
1> [
1> _Ty=double,
1> Container=std::vector>
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(54): note: see reference to class template instantiation 'boost::arg<9>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(53): note: see reference to class template instantiation 'boost::arg<8>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(52): note: see reference to class template instantiation 'boost::arg<7>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(51): note: see reference to class template instantiation 'boost::arg<6>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(50): note: see reference to class template instantiation 'boost::arg<5>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(49): note: see reference to class template instantiation 'boost::arg<4>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(48): note: see reference to class template instantiation 'boost::arg<3>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(47): note: see reference to class template instantiation 'boost::arg<2>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/bind/placeholders.hpp(46): note: see reference to class template instantiation 'boost::arg<1>' being compiled
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/detail/indexing_suite_detail.hpp(605): warning C4267: '+=': conversion from 'size_t' to 'long', possible loss of data
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/detail/indexing_suite_detail.hpp(590): note: while compiling class template member function 'void boost::python::detail::slice_helper,Index>,Data,Index>::base_get_slice_data(Container &,PySliceObject *,Index &,Index &)'
1> with
1> [
1> Container=std::vector>,
1> DerivedPolicies=boost::python::detail::final_vector_derived_policies>,false>,
1> Index=unsigned __int64,
1> Data=double
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/detail/indexing_suite_detail.hpp(695): note: see reference to function template instantiation 'void boost::python::detail::slice_helper,Index>,Data,Index>::base_get_slice_data(Container &,PySliceObject *,Index &,Index &)' being compiled
1> with
1> [
1> Container=std::vector>,
1> DerivedPolicies=boost::python::detail::final_vector_derived_policies>,false>,
1> Index=unsigned __int64,
1> Data=double
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/indexing_suite.hpp(256): note: see reference to class template instantiation 'boost::python::detail::slice_helper,Index>,Data,Index>' being compiled
1> with
1> [
1> Container=std::vector>,
1> DerivedPolicies=boost::python::detail::final_vector_derived_policies>,false>,
1> Index=unsigned __int64,
1> Data=double
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/indexing_suite.hpp(253): note: while compiling class template member function 'void boost::python::indexing_suite::base_delete_item(Container &,PyObject *)'
1> with
1> [
1> Container=std::vector>,
1> DerivedPolicies=boost::python::detail::final_vector_derived_policies>,false>
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/indexing_suite.hpp(186): note: see reference to function template instantiation 'void boost::python::indexing_suite::base_delete_item(Container &,PyObject *)' being compiled
1> with
1> [
1> Container=std::vector>,
1> DerivedPolicies=boost::python::detail::final_vector_derived_policies>,false>
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/vector_indexing_suite.hpp(50): note: see reference to class template instantiation 'boost::python::indexing_suite' being compiled
1> with
1> [
1> Container=std::vector>,
1> DerivedPolicies=boost::python::detail::final_vector_derived_policies>,false>
1> ]
1>C:\ThirdParty\1.8.0\win64-vc14\boost\include\boost-1_63\boost/python/suite/indexing/detail/indexing_suite_detail.hpp(619): warning C4267: '+=': conversion from 'size_t' to 'long', possible loss of data
```

I'm using Boost 1.63, VisualStudio 2017 15.5.4.