Use C++17's std::filesystem::path to represent paths about libloot HOT 3 CLOSED

AppVeyor have now updated to VS 15.7, so there's no blocker on this now.

from libloot.

While writing tests for this change, I've realised that the single-argument path constructor that takes a string doesn't handle character encoding as boost::filesystem::path did, as it assumes that a char-based string uses the native narrow encoding, which isn't UTF-8 on Windows, so paths get mangled if constructed that way.

There is an overload that takes string, locale, which uses the given locale to perform character encoding conversion. This works correctly if passed a locale that's been created with Boost.Locale to have UTF-8 support, but otherwise fails. We currently set the global locale using Boost.Locale, so that's not much of an issue, but that does mean relying on gobal state.

The best option is to use std::filesystem::u8path() to construct paths from UTF-8 strings. This doesn't rely on supplying a locale, it only requires the input string to be UTF-8, which the API already expects of all its strings.

Unfortunately, this means I have to go through the existing code looking for everywhere it uses the path string constructor, including when it's done implicitly, like when passing as function arguments and appending as path components. There's a lot of code to go through...

from libloot.

Done as of fa37dc6.

from libloot.