Fix use of `macroexpand-all` in `loopy-iter` and `loopy`.

[okamsn]

We should always be passing an environment to the function, otherwise the environment is reset to nil, which breaks macros like cl-flet.

• Stop doing top-level and sub-level expansion with different functions.
  • Instead, add loopy-iter--level.
  • Remove loopy-iter--sub-level-expanders, loopy-iter--top-level-expanders, loopy-iter--macroexpand-sub, loopy-iter--macroexpand-top, loopy-iter--keyword-expander-sub, and loopy-iter--keyword-expander-top.
• Pass the macro temporary environment in all places where we didn't before:
  • loopy-iter
  • loopy
  • loopy-iter--parse-at-command
  • loopy-iter--opt-accum-expand-val
• Add a loopy--with-protected-stack and loopy--bind-main-body.
• Add tests.