Add support for member pointer casting

[Lancern]

Given class types Base and Derived, assume Base is a public non-ambiguous non-virtual base class of Derived, there are 4 valid ways to cast relevant member pointers:

• Casts on pointer to data members:
  • Implicit cast from T Base::* to T Derived::*
  • Explicit cast from T Derived::* to T Base::*
• Casts on pointer to member functions:
  • Implicit cast from Ret (Base::*)(Args) to Ret (Derived::*)(Args)
  • Explicit cast from Ret (Derived::*)(Args) to Ret (Base::*)(Args)

This issue tracks the implementation status of these 4 cast types.