Accept std::basic_string with custom allocators in the API instead of enforcing std::string

[mp-t]

The ImageMagick interface accepts a std::string in several functions. For example the Magick::Image constructor.

It would be useful if these functions would be templated to allow a std::basic_string<char, T>.

This would allow strings with custom allocators to work seamlessly with ImageMagick.

[dlemstra]

I don't know if we can do that without breaking the existing API/ABI. Do you have some more info about this?

[leha-bot]

We could "templatize" these parts of library (like std::basic_string does), make current classes as derived and we could check ABI with this tool: https://github.com/lvc/abi-tracker
I also think that templatized API should be in namespace "Experimental" or some semantically similar and we could make it as main in IM 8.