Add vm_thread::call_frame struct / member variables.

Needed for storing call frame info, obviously.
Remove call frame instructions.

Thought it would be useful, in retrospect: not so much, since there's
no way of inspecting the callframe in a way that makes it useful.

Instead, the call frame stuff will still be useful for working with the
defer/join instructions, but not yet.
Add down/up/drop_frame member function implementations.
Replace exec_call() use with up/down_frame calls and avoid nested runs.