8086 assembler

Work in progress. Load with "750 LOAD".

Mnemonics are followed by argument types. For example, MOVri,
moves 8-bit immediate to 8-bit register.

'r' = 8-bit register           'x' = 16-bit register
'i' = 8-bit immediate          'I' = 16-bit immediate
's' = SREG register

Mnemonics that only have one signature (for example INT,) don't
have operands letters.

For jumps, it's special. 's' is SHORT, 'n' is NEAR, 'f' is FAR.