Our class-creation code is getting pretty unruly, since there
are four places we can create new classes:
1) extension classes
2) Python-defined classes
3) builtin-defined classes
4) classes created during bootstrapping

Then, there are multiple different sets of initialization functions
that can be called, each of which initializes different sets of the
class attributes.  It's not easy to tell what a given initialization
function should actually initialize.

This change just cleans this up slightly: it renames PystonType_Ready
to commonClassSetup to hopefully be more clear about its goal, as well
as adds a BoxedClass::finishInitialization() function.

It also moves some work out of the BoxedClass() constructor, since
this causes issues because it happens before the BoxedHeapClass()
constructor runs, which sets certain class slots that affect other
parts of class initialization.

Things are still pretty complicated but this should help with the
multiple inheritance changes that are coming.