We seem to be spending a fair amount of time doing unnecessary work
for simple calls like boxInt and createList, which are generated
by irgen and reduce to calling new BoxedInt / BoxedList.  The
operator new calls tp_alloc, so we get some indirect function calls,
and then tp_alloc does some checking about its caller, and then we
check to see what size object to create, and how to initialize it.

I created a DEFAULT_CLASS_SIMPLE macro to go with DEFAULT_CLASS,
that should help with these things.  I (manually) inlined all of those
functions into the operator new.

I also moved the small arena bucket selection function (SmallArena::alloc)
into the header file so that it can get inlined, since the allocation size
is often known at compile time and we can statically resolve to a bucket.

Putting these together means that boxInt and createList are much tighter.