ABI - AVM Support - selector and dig/bury

[chaihoang]

• Have a pseudo opp similar to ADDR to calculate method selector
• Revisit dig/bury for easier stack manipulation

[jannotti]

Here are proposed ops to give us better stack manipulation

cover n : remove TOS, and place it down the stack such that n elements are above it
uncover n: inverse of cover (this is also like dig, but removes the deep element)