The name mangling needs revamp, for both correctness and overall efficiency (do not allocate as much to construct the mangled names; and make the mangled names shorter). See #2175.

• Do we need to include the namespaces in the typenames? What if we instead just use the type simple name; and disambiguate it if there is conflict - similar to what we do for methods.
• Use shortcut for the system module or frequently used system types. E.g. The reference to System.String type today is something like: System_Private_CoreLib__System__String. What if it is just something like #String?
• CppCodeGen and native codegen name mangling can be quite different
• Store the mangled strings as UTF8
• Robustness against collisions with built-in C++ identifiers or our internal identifiers