conditional_asm

- Nested conditionals work!
- Statement labels on conditionals are inserted into the assembler's
  symbol table. Why one would want to do this is questionable,
  especially putting a statement lable on an 'else', but it is
  supported.
- Pretty printer does something useful! (Actually, right now, it's not a
  big stretch to generate the assembler listing. The only problem is
  preserving the whitespace formatting of inline [as opposed to
  statement-ending] comments.)