golang_str: Invoke bytes/unicode methods via zbytes/zunicode

GPython will patch builtin bytes and unicode types.
zbytes and zunicode will refer to original unpatched types.
We will use them to invoke original bytes/unicode methods.

NOTE we will test against bytes/unicode - not zbytes/zunicode - when
inspecting type of objects. In other words we will use original
bytes/unicode types only to refer to their original methods and code.