add vtable overrides to Winxed class

[bubaflub]

adding vtable overrides will allow us to do things convenient but slow like addition between to GMP Integers without having to call mpz_add

the syntax is pretty simple in Winxed, but type checking will have to happen so we can choose the correct function to add - mpz_add or mpz_add_ui and so forth.

[bubaflub]

get_string override was in 4ce91a5, get integer in 03673fe.

[bubaflub]

658b5e6 has all of the overrides with tests.