Use stackalloc directly into Span (dotnet/corefx#24212)

Use stackalloc directly into Span (dotnet/corefx#24212)

* Use stackalloc directly into Span

We have a few places in corefx where we were stackalloc'ing into a temporary pointer and then creating a Span from that.  Now that we're using a C# compiler which supports stackalloc'ing directly into span, use that feature instead.

* Address PR feedback

Commit migrated from https://github.com/dotnet/corefx/commit/a7f6f470cb2c4cdaafdc3ad85e2520992a8db265