[mypyc] Fix vtable construction for deep trait inheritance

[VaggelisD]

Fixes mypyc/mypyc#1189

The core issue: compute_vtable iterates cls.traits (direct parents only) instead of the full trait MRO, so when an intermediate trait is empty, grandparent trait methods are never added to the vtable.

The difference between cls.traits and all_traits for the issue's repro is the following:

Class Child
-----------

 - cls.traits:
    •  Middle  --> {}

 - all_traits:
    •  Child   --> {'derived': <FuncIR Child.derived>}
    •  Middle  --> {}
    •  Base    --> {'value': <FuncIR Base.value>}