Add support for `boost::string_view` into `container::basic_string`

[mclow]

Conversion from a basic_string to a string_view, and make all the comparisons work.

Ion - if you like this, I'll work up some tests, too.

[igaztanaga]

Thanks Marshall. I need to think about it, maybe a templated solution might be a better approach (interoperability with any "view") to avoid injecting boost::string_view dependencies and to support std::string_view and other user-defined views. In any case, your patch is a good starting point.