tvix-eval: rename "call frame" to "bytecode frame"

Suggested by amjoseph on cl/8226, this is probably a better name as "call frame" is kind of ambiguous.

However, while cl/8104 is still in flight, I'd rather not change something that will require touching a bunch of different areas. For that reason this bug is intended to track that we want to do that renaming (in at least the vm module and in docs/vm-loop.md)