Split OpSpec and OpImpl

[antocuni]

This is a no-op renaming of things.
Before:

• OPERATORs returned W_OpImpl
• typecheck() turned a W_OpImpl into a W_FuncAdapter
• however, the return type of typecheck() was W_Func
• astframe and doppler had to pretend to deal with arbitrary W_Funcs when calling their w_opimpl, thus adding complexity and indirection.

This PR creates two distinct concepts:

• OpSpec: the old OpImpl: it's what the user returns, and it's a "high-level, non-typechecked" description of how to implement an op. Usually, "call this func with these args"
• OpImpl is the old W_FuncAdapter, to underline that it's the concrete way to execute an op
• OpImpl are executed by calling w_opimpl.execute() as opposed to vm.fast_call(w_adapter) as it was before