Spring time cleanup and style improvements

This removes unused methods such as find_super_instructions. The style
changes are mostly moving down variable definitions closer to where they
are used and using consistent style for similar but slightly different
methods.

We also move the private constructor for Object up one level to
ObjectHeader so we can't create those as C++ object
Use unified lookup logic for vm_check_(super)_callable

Instead of another place of having some implementation of method lookup,
let's use the unified place we have so we ensure always consistent and
correct results.