Refactor Pulley's interpreter loop (#9629)

* Refactor Pulley's interpreter loop

* Define loop-over-match and loop-with-tail-calls in separate files to
  make it more clear which is in which (and less `#[cfg]`)
* Move per-opcode handlers to `interp.rs` outside of a macro invocation
  to get better native editor support (e.g. formatting, hints, etc).

This is roughly intended to be perf-neutral but we don't have many
automated benchmarks yet for Pulley so it's intended to profile later as
well.

* Model interpreter as trait implementation

* Add more comments

Author: alexcrichton