Move Marshal class to S.P.CoreLib (dotnet/corert#7502)

Move Marshal class to S.P.CoreLib (dotnet/corert#7502)

After this change, CoreRT only uses S.P.Interop to host the interop hashtable readers. Those can't move to CoreLib because they call into the type loader.

I didn't touch ProjectN's version of Marshal and I left the `PInvokeMarshal` class in place to help with that. I guess we'll need to make a call at some point on what to do with S.P.Interop since without MCG the code in it is kind of useless.

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>