The following words allow you to assemble z80 binaries. Being
Forth words, opcode assembly is a bit different than with a
typical assembler. For example, what would traditionally be
"ld a, b" would become "A B LDrr,".

H@ offset at which we consider our PC 0. Used to compute PC. To
have a proper PC, call  "H@ ORG !" at the beginning of your
assembly process.

Labels are a convenient way of managing relative jump
calculations. Backward labels are easy. It is only a matter or
recording "HERE" and do subtractions. Forward labels record the
place where we should write the offset, and then when we get to
that point later on, the label records the offset there.