Add stack-relative operations

[BigEd]

At least LDA and STA. From the 65816 we have

• A3 lda d,S
• 83 sta d,S
  and, less crucial, support for pointers on the stack
• B3 lda (d,S),Y
• 93 sta (d,S),Y
  where d is a constant displacement operand.

(We might not want to use column 3.)