Enable use of `boost::adaptors::tokenized` with std::regex

A superficial look at https://github.com/boostorg/range/blob/develop/include/boost/range/adaptor/tokenized.hpp shows that the only reason, why there is a hardcoded dependency on boost::regex is because `regex_constants::match_default` is used as a default parameter in some of the functions. Everything else is already templateized to work with anything regex like and should work with std::regex just as well.

If I get the go-ahead from you I'd like to replace those functions with two functions: One that doesn't have the parameter at all and one that takes the parameter as a tempate argument (the functions are already templated anyway). Of course I'll also have to adapt some of the internals, but all changes would be localized to that file (and the tests of course).

This would have two advantages:

- boost::adaptors::tokenized would work with std::regex not only with boost::regex
- It would remove the sole dependency on Boost.Regex from Boost.Range