Implement the `mro_lookup` primitive

[omelancon]

Naming class_getattr is an historical mistake since it takes an object, not a type, as first argument. Not having a primitive to execute a lookup directly on a type has become a problem in #2.

We should implement a mro_lookup primitive. Doing class_getattr could then be written as mro_lookup(type(x), "__method__"). This is both more expressive and more accurately named.

[tekknolagi]

We called our version of this _object_type_getattr but were similarly a little confused about how to name the thing.