Add UTF-8 match algorithm for FileSystemName?

[JeremyKuhne]

A UTF-8 implementation of FileSystemName.MatchesSimpleExpression would allow matching Unix/Mac data without having to first convert to UTF-16. Something like this perhaps:

namespace System.IO.Enumeration
{
    // name is UTF-8
    public static bool MatchesSimpleExpression(ReadOnlySpan<char> expression, ReadOnlySpan<byte> name, bool ignoreCase = true);
}

This could potentially improve performance. We currently convert from UTF-8 into a temporary UTF-16 buffer on the stack- it may also be that improvements here don't register against the cost of I/O. https://github.com/dotnet/corefx/blob/master/src/System.IO.FileSystem/src/System/IO/Enumeration/FileSystemEntry.Unix.cs#L88-L103

If anyone has a particular interest in this they are welcome to try and implement. If we find there is, in fact, a measurable benefit we can look at making a formal API proposal at that point.

[JeremyKuhne]

Triage: We should wait for the UTF-8 string work to finish to finalize on what we do here.

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