There are now no known cases where the compiler package computes a
stack depth lower than the one computed by the builtin compiler.  (To
achieve this state, we had to fix bugs in both compilers :-).

The chief change is to do the depth calculations with respect to basic
blocks.  The stack effect of block is calculated.  Then the flow graph
is traversed using breadth-first search to find the max weight path
through the graph.

Had to fix the StackDepthTracker to calculate the right info for
several opcodes: LOAD_ATTR, CALL_FUNCTION (and friends), MAKE_CLOSURE,
and DUP_TOPX.

XXX Still need to handle free variables in MAKE_CLOSURE.

XXX There are still a lot of places where the computed stack depth is
larger than for the builtin compiler.  These won't cause the
interpreter to overflow the frame, but they waste space.