Unnecessary injection of std::string dependency for BOOST_NO_EXCEPTIONS

When using safe_numerics in embedded system (kernel mode code), where exceptions are disabled, uncessary dependency on std::string is created (std::exception and friends, as well). Currently I have patched this out locally via ``#ifdef BOOST_NO_EXCEPTIONS``, however maybe there might be better solution ?

https://github.com/boostorg/safe_numerics/blob/13ca3d6dd36db1aac2d6b5caca2c281d15c881ad/include/boost/safe_numerics/exception.hpp#L118
https://github.com/boostorg/safe_numerics/blob/13ca3d6dd36db1aac2d6b5caca2c281d15c881ad/include/boost/safe_numerics/exception.hpp#L190

Start at include/boost/safe_numerics/exception.hpp around lines 118 and 190, focusing on how BOOST_NO_EXCEPTIONS affects the std::string and std::exception dependencies. Reproduce a no-exceptions embedded or kernel-mode build, then verify that the dependency is absent without changing behavior for exception-enabled builds.

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