Support char16_t and char32_t sequences

[igoloe]

Contrary to what is stated in the docs, source code does no seem to have support for utf16 an utf32 char sequences. At least specialization of path_traits for is_pathable is missing and I assume for convert as well. I am not sure, what the internal decisions are, maybe its just postponed?

[Lastique]

Boost.Filesystem does support UTF-16 and UTF-32 through wchar_t. charN_t are the newer additions to C++ which are not supported yet.

[mbs-c]

Could the documentation be updated to reflect that? Right now, it contains references to char16_t, char32_t, u16string and u32string in various places:

• https://www.boost.org/doc/libs/1_74_0/libs/filesystem/doc/reference.html#Introduction
• https://www.boost.org/doc/libs/1_74_0/libs/filesystem/doc/reference.html#class-path
• https://www.boost.org/doc/libs/1_74_0/libs/filesystem/doc/reference.html#path-Requirements