Remove unsafe from method
Call SigComparer(opts) ctor instead of def ctor
Resolve MemberRef now defaults to also searching base types

Fixes #16
Fixes

- AssemblyNameComparer doesn't return an asm with a different name
  if one of the inputs is null
- AssemblyResolver verifies that the assembly isn't null
Merge branch 'master' of https://github.com/0xd4d/dnlib