Refactor reg_save
Cosmetic refactoring

Not including sexp#swap_element(), the ugly beast of the file.
Split s:current_top_form_bracket() into two functions

One for each algorithm
Decompose sexp#stackop into capture and emit fns
Use try/catch/finally over for/break

It's an even better goto/label than for/break
Decompose sexp#swap_element

Still not pretty, but no longer shameful
Merge branch 'refactor'

* refactor:
  Decompose sexp#swap_element
  Use try/catch/finally over for/break
  Decompose sexp#stackop into capture and emit fns
  Split s:current_top_form_bracket() into two functions
  Cosmetic refactoring