Allow negative lexical scope levels in the LEXICAL_ADDRESS opcode, and allow UNDEF to be used to indicate current lsl. Also fix a bug which was preventing eg 'SEG_START','PUSH','SEG_END',1,'RETURN','SEG_END','EXEC' from being passed - i.e. make the CPU fetcher aware that even in deferred mode, PUSH is essentially a means to escape whatever follows. Discovered a subtlety wherein due to the behaviour of SEG_START, opcodes in deep descendents will get pushed to the stack multiple times - one for each of their ancestor segments. literal lexical addresses get realised on the first such push, but the segment doing that realisation may still be a substantial ancestor. The subsequent pushes essentially become noops.