Fixes to static field offsets (#699)

Fixes to static field offsets (#699)

* Fixes to static field offsets

1) Stop counting offsets for RVA fields (this messes up the offsets of all statics)
2) Small fix to the size computation of nonGC statics (we were counting some GC statics as non-gc statics)
3) Bringing a small optimization we had with crossgen1 (safe static byref return)
4) Tiny micro-optimization: avoid allocation while looking up a field in one of the Typesystem hashtables (introduced with determinism work)

* Count the sizes of RVA statics in module field layout to match coreclr TypeSystem