pegbox: debug: add runtime BUG_ONs

BUG_ON(r) panics if r!=0, BUG_UNLESS(r) panics if r==0.  r may be any 8-bit
 operand acceptable to 'LD A,r'; this includes (HL) and A itself.
BUG_ON_HL and BUG_UNLESS_HL are similar tests for HL (*not* (HL), for which
 use BUG_ON(HL)).
All four of these will clobber the Accumulator (except when r is A).