Improve accessibility in debugging

[mingxwa]

Motivation

Member functions of proxy cannot be called from a debugger, making it difficult to debug with a value of proxy from client code.

Changes

• Added a helper type details::symbol_guard to guide symbol generation in a debug build (when NODEBUG is not defined).
• Enabled symbol generation for proxy::operator-> and proxy::operator*.
• Enabled symbol generation for all the pre-defined dispatch types and macros that generates member functions, including operator_dispatch, conversion_dispatch, PRO_DEF_MEM_DISPATCH, PRO_DEF_FREE_AS_MEM_DISPATCH and the dispatch type generated for basic_facade_builder::add_facade<F, true>.
• Refactored the implementation of proxy accordingly to ensure template instantiation happens at the right spots.
• Moved some types out of an anonymous namespace in unit tests to avoid generating warnings of unused variables in a debug build.
• Removed some warning suppressions in unit tests accordingly.

Verification

Build the following code with MSVC in debug mode (without using the value of proxy in the code):

#include <string>

#include "proxy.h"

PRO_DEF_FREE_AS_MEM_DISPATCH(MemToString, std::to_string, ToString);

struct Stringable : pro::facade_builder
  ::add_convention<MemToString, std::string() const>
  ::build {};

int main() {
  int v = 123;
  pro::proxy<Stringable> p = &v;
}

Set a breakpoint at the end of main and inspect the value of p in the debugger. The debugger invokes the correct function for me.