Change view_from_string to be more 2 phase name lookup friendly

[dunhor]

The original issue dates waaaaay back to issue #8 where str_concat was technically not two-phase-name-lookup clean since function overload sets only consider the functions that have been declared at the time of use, so all of the other str_raw_pointer overloads should not be considered for overloads. So this is an issue I've been wanting to tackle for some time.

When syncing with the OS, there was a conflict when calling str_concat with both std::filesystem::path and winrt::hstring since both are implicitly convertible to both std::wstring and wil::zwstring_view. Since I need to address those errors, I figured I might as well tackle both.

This changes it so that view_from_string delegates out to a class template that can be specialized, which doesn't have the same problems as function overloads. The negative consequence is that str_concat can no longer be used with types implicitly convertible to std::wstring, however upon further inspection I only found one reliance on this and it was for std::filesystem::path, and fixing that usage is goodness anyway since the implicit conversion makes a copy (calling .native() is better).

Fixes #8