This allows dynamically indexing into tuples. IRGen not yet
implemented.
I think I'm going to need a type_refine_addr instruction in
order to handle substitutions into the operand type that
eliminate the outer layer of tuple-ness. Gonna handle that
in a follow-up commit.
Having added these, I'm not entirely sure we couldn't just use
alloc_stack and dealloc_stack. Well, if we find ourselves adding
a lot of redundancy with those instructions (e.g. around DI), we
can always go back and rip these out.
