Fix some stack management issues.
Scatter some stack spilling operations.

Get us towards enforcing "stack empty on call", which will simplify
using invokedynamic for methods but, more importantly, is a prereq for
coroutine support which Rakudo will need.
Basic invokedynamic handling for methods.

Gets the dispatch logic out of the generated code, which in the case
of the method-call heavy QAST compiler knocks a third off the size of
the output. Not really faster yet; need to stack a PIC-ish thing up in
the callsite for that.