extend: add command '!' to jump to user-given memory cell
'!', or InDP:
- prompts the user for a memory-cell location
- attempts to parse an Int
- If the parse fails, carry on (DP+1)
- If the parse succeeds, (DP+parse)
No need to check for out-of-bounds.
'!', or InDP:
No need to check for out-of-bounds.