This thingie was inspired by https://abagames.github.io/z80-repl/index.html
Type your assembly instructions when in REPL mode, and they will be assembled at the current PC (but not automatically executed), and the PC advanced.
Instructions involving indirection + offset, such as (ix+4), bit instructions, and selected extended instructions are not implemented yet.
No IN/OUT either.
Cursor keys - move the PC around
PgUp, PgDown - move the 256 bytes window downwards (+100h) or upwards (-100h)
F9 - Set PC to 0
F10 - Step over the instruction at PC
F1 - Switch to REPL mode
F2 - Run from PC
F3 - Save to snapshot.mem
F4 - Restore from snapshot.mem
\> nnnn - set PC to nnnn
RUN - Run from PC
Escape - quit