Added boolean and comparison operators

Added VM instructions GR, LR, GER, LER, EQR, NER, ANDR, ORR, XORR
and NOTR. (Leaving the immediate operand variants until #64 is done,
as we're running out of opcodes!)

Added a2c_GetOpOrChar() to handle the C style two character
comparison operators.

Added compiler logic to generate and dump the new instructions.

Minor lexer cleanup; a2_UngetChar() instead of messing directly
with the read position.