Large dict literals would crash the bjit since it would try to allocate
scratch space to store all of the keys+values on the stack.

This is also, in a small way, detectable to the user, since we would evaluate
all subexpressions before doing any dict operations (which could trigger
__hash__ and __eq__ calls).  I started working on this, but it looks
like it's not just an issue in the JIT tiers, but it's also encoded in
the CFG phase as well.  Punting on that for now since it's not a refcounting issue.