Remove MemoryFS dependency on Windows Path implementation

[gderham]

Hi, long term fan of language-ext 😄

I ran all LanguageExt.Tests tests on macOS (.NET 7.0) and some fail, mostly due to MemoryFS using Path.IsPathRooted(), Path.DirectorySeparatorChar etc with error:

System.IO.IOException : Path not rooted: C:
   at LanguageExt.Sys.MemoryFS.ParsePath(String path) in /Users/_/code/language-ext/LanguageExt.Sys/MemoryFS.cs:line 51

In this PR I've modified MemoryFS and MemoryFSTests to behave the same independent of the host OS, and therefore enable the tests to pass. The replacement methods aren't 100% realistic but I hope acceptable for this usage. There remains a dependency on System.IO for some exceptions and other types.

MemoryFS use of Windows file paths is unchanged so there is no breaking change.

Thanks

[louthy]

Thanks for this, I'll have a look in more detail next week. My initial thoughts on a quick scan through is that probably LanguageExt.Sys needs its own Path trait like the other traits (with platform specific implementations). We can then do Path<RT>.* to get to the injected path-functionality.

I had naively thought that Path wouldn't have side-effects, I should never have given MS the benefit of that doubt!