Implement pointer-to-member support

[mingxwa]

Changes

• Added pointer-to-member type dispatch_ptr and consteval function template get_dispatch_ptr() for proxy.
• Added overloads of proxy::operator->*.
• Added another unit test case for this change.

Here is a comparison table for the syntax of pointer-to-member in direct C++ types and proxy:

Semantics	Direct C++ type T (value denotes a value of T)	proxy type P (value denotes a value of P)
Pointer-to-member type (function)	R (T::*)(Args...)	P::dispatch_ptr<R(Args...)>
Pointer-to-member type (field)	M T::*	N/A
Acquire pointer-to-member (function template)	&T::member<Args...>	P::get_dispatch_ptr<Args...>
Invoke member function with pointer-to-member ptr	(value.*ptr)(args...)	(p->*ptr)(args...)

Note that we cannot use pointer-to-member in multiple inheritance today in MSVC due to a bug in the compiler implementation. Therefore, we fallback the implementation for MSVC to C-style offsetof. Interestingly, offsetof won't compile in GCC or clang in our scenario (probably we hit some limitations in macro expansion), but we have not investigated further while pointer-to-member works as expected in the two compilers.

Resolves #107