Improve boolean and some iteration commands, unify tests

[okamsn]

• Make it an error to use the same iteration var for multiple iteration commands.

  • Add loopy--other-vars for the set commands.
    These variables don't work exactly like iteration or accumulation variables,
    so it needs it's own group.

• Improve boolean commands by making their behavior more like accumulation
  commands.

  • Switch to loopy-result instead of a variable based on the loop name.
  • Use accumulation variables instead of iteration variables.
  • Make boolean commands exit via leave.
  • Check the compatibility of the used boolean commands during expansion,
    like we already do with accumulation commands.
  • Add :into to the boolean commands.

• Improve cons, cycle, iter, numbers, and seq-index, by defaulting
  to avoiding updating the iteration variables during the loop. This lets us
  avoid indirect variables and produce faster code. This can be overridden
  using the with (or without) special macro arguments.

  • For cycle in particular, we now allow it to do the opposite.
  • Add predicates loopy--with-boundp and loopy--command-boundp.

• Update Org doc and changelog with these changes.

[okamsn]

Also, add destructuring to iter, which we apparently forgot to do.

[okamsn]

• Make the expanded test prefixed by what is tested. So, instead of
  one test list that tests both loopy and loop-iter, it should expand
  to multiple tests loopy/list, iter-bare/list, and
  iter-keyword/list. The name will be more informative when things fail.
• Instead of relying on generic errors, try to use more defined errors from
  define-error. We can then name these errors in the tests for more
  robustness.