We already supported changing the values, but not the number
of them.  The main trickiness here is
- We had been assuming that the number of defaults was immutable,
  so I had to find the places that we used it and add invalidation.
- We assumed that all functions based on the same source function would
  have the same number of defaults.

For the first one, I found all the places that looked at the defaults array,
which should hopefully be all the places that need invalidation.

One tricky part is that we will embed the num_defaults data into code produced
by the LLVM tier, and we currently don't have any mechanism for invalidating
those functions.  This commit side-steps around that since the only functions that
we can inline are the builtins, and those you aren't allowed to change the defaults
anyway.  So I added a "can_change_defaults" flag.

For the second part, I moved "num_defaults" from the CLFunction (our "code" object)
to the BoxedFunction (our "function" object), and then changed the users to pull
it from there.