A few more Enum perf improvements (dotnet/coreclr#21284)

A few more Enum perf improvements (dotnet/coreclr#21284)

* A few more Enum perf improvements

I started out to just remove the unnecessary boxing allocation that happens when you do enum.ToString("D") or, if the value doesn't map to a named enum entry, just enum.ToString() (there's an internal boxing of the value that happens in addition to any boxing that happens of the enum itself).  As part of that, I added a ValueToString method that replaces the GetValue().ToString() calls that appear several times, and in writing that ValueToString, I opted to avoid pinning, instead using Unsafe.As.  Once I did that, I then removed pinning everywhere else it was being done, standardizing on Unsafe.As.

* Address PR feedback

Commit migrated from https://github.com/dotnet/coreclr/commit/7972722e3a3788fe96056d0a5398f90466588b8f