SqlClient spanify netcore guid writes (dotnet/corefx#34390)

SqlClient spanify netcore guid writes (dotnet/corefx#34390)

* add netcore specific stackalloc paths for guid writes

* remove temp byte array allocation in favour of direct span

* remove reliance on span zeroing on allocation

* change to branch and explain why clear is needed

* split netstandard and netcore #ifdef regions out into conditionally included files
revised WriteGuid, WriteFloat and WriteDouble to use new Get*Bytes functions

* convert single line netcoreapp variants to expression body form

* rename Get*Bytes to Fill*Bytes

Commit migrated from https://github.com/dotnet/corefx/commit/36037e16523a947aa2de05eca587a6a3bd5d18b5