Refactor and combine all FileType structs in yanix

[kubkon]

This commit does a bit of everything: refactors bits here and there,
fixes a bug discovered in another #701, and combines all structs that
we used in yanix and wasi-common crates to represent file types
on *nix into one struct, yanix::file::FileType.

Up until now, in yanix, we've had two separate structs used to
represent file types on the host: yanix::dir::FileType and
yanix::file::SFlags (well, not quite, but that was its main use).
They both were used in different context (the former when parsing
dirent struct, and the latter when parsing stat struct), they
were C-compatible (as far as their representation goes), and as it
turns out, they shared possible enumeration values. This commit
combines them both into an idiomatic Rust enum with the caveat that
it is now not C-compatible, however, I couldn't find a single use
where that would actually matter, and even if it does in the future,
we can simply add appropriate impl methods.

The combine yanix::file::FileType struct can be constructed in two
ways: 1) either from stat.st_mode value (and while we're here,
now it's done correctly according to POSIX which fixes the bug mentioned
in VFS impl PR #701), or 2) from dirent.d_type value. Also, since we now
have one struct for representing both contexts, this cleans up nicely
a lot of duplicated code in host module.

cc @iximeow Hopefully, this fixes the bug you mentioned in #701!

[kubkon]

I'm gonna go ahead and merge this to not thwart progress in #701, but if anything is amiss, please feel free to reopen, revert, or file an issue.