Reduce Unix enumeration allocations (dotnet/corefx#26942)

Reduce Unix enumeration allocations (dotnet/corefx#26942)

* Reduce Unix enumeration allocations

This change factors out the FileStatus access into a helper struct and adds overloads
for the stat imports  that allow passing a span.

The next steps:

- Handle errors manually
- Skip using safehandle
- Look for further opportunities around UTF-8/16 conversion

* Address feedback

Shuffle code around a bit in FileInfo for clarity. Also shift directory determination
to FileSystemEntry to avoid allocation on links.

Commit migrated from https://github.com/dotnet/corefx/commit/c64171e379090b372fe232ec3f27a57ae838406c