Some old regalloc cleanups
Add a basic "register allocator" that just loads/stores everything on the stack. Just a temporary hack to unblock other compiler work before the real allocator is ready.
Add a basic SSA construction pass
Add list.index method
dumb_regalloc cleanups/minor fixes
SSA construction improvements/fixes: support parameters and labels
Remap instruction IDs instead of the hacky "dup" construct
Add a modification syntax for succinctly expressing setitem/setattr
Use modification syntax in some places