`path` methods taking `codecvt` should perform `char`<->`char` code conversion

[Lastique]

Currently, constructor/assignment/appending methods that accept codecvt_type arguments only perform character code conversion when the source character type does not match path::value_type. C++ standard requires (e.g. here and here) that when both path::value_type and source value type are char the conversion is performed twice: first, from source encoding to wchar_t using the supplied codecvt_type facet, then from wchar_t to native path character encoding using path::codecvt().