Various small fixes

- Use GetSafeEnumerable() instead of IterateAll()
- Remove a local
- Dispose of MD stream if input is null
- Remove useless Dispose() call
- Return cached pdb doc
- pdbDocs field is now readonly
- Remove useless braces
- Fix indentation
Add ARM64 machine type
Add asm name in thrown resolve exceptions
Add option to override corlib asm ref
Add Is{Setter|Getter|Other|AddOn|RemoveOn|Fire} MethodDef props
Add IsPrimitive to IType