Let the attributes live at various offsets in an object.

Similar to the location of the dict attribute in CPython,
the 'hc attrs list' in Pyston will end up being located at
different offsets, depending on the class that's being subclassed
from.

This commit removes the HCBox class, which assumes that it always lives
at the same offset and so can be statically described with a C++ class,
and instead uses a runtime lookup.

This should hopefully make things a fair bit closer to supporting inheritance.