LWT: Support for wide/unicode strings

[Flamefire]

Currently BOOST_TEST_EQ(std::u16string{}, std::u16string{}) fails to compile. Similar for wstring, u32string and u8string.

Additionally support for char8_t comparison is missing.

While implementing this I'd suggest to try printing the ASCII subset of those directly similar to the char handling. The current differences in output are a bit confusing:

: test 's[0] == s2[1]' ('1' == '2') failed in function 'void test(const Ch *) [Ch = char]'
: test 's[0] == s2[1]' ('49' == '50') failed in function 'void test(const Ch *) [Ch = wchar_t]'
: test 's[0] == s2[1]' ('49' == '50') failed in function 'void test(const Ch *) [Ch = char16_t]'
: test 's[0] == s2[1]' ('49' == '50') failed in function 'void test(const Ch *) [Ch = char32_t]'