Add missing features

[jjrv]

Emscripten target is still missing one feature:

• Inheritance between C++ classes and automatic upcasting.
• Const and non-const pointers as argument and return types.
• Const and non-const references as argument and return types.
• Smart pointers as argument and return types.
• Conversion between JavaScript arrays and C++ vectors.
• Plain functions (like static methods but outside any class).
• Destroy method, freeing the C++ instance and invalidating the wrapper object's internal pointer.
• Support passing int64_t and uint64_t losslessly to a bignum object with hi, lo parts and a toString method.

[jjrv]

Multiple inheritance is almost done. It makes overload resolution much more annoying, so overload support has been turned into a project and postponed until remaining issues are resolved.

[jjrv]

Now these are finally done!