Add ALT stack

[bobbinth]

Currently, all programs execute against a single stack. Adding an alt stack would enable:

1. More expressive programs where a program could cache values on the alt stack
2. A relatively simple way of implementing full random access memory (this would also require operation mirroring).

Adding of ALT stack (without mirroring) would require 2 new instructions:

• TOALT - to move a value from the main stack to the alt stack
• FROMALT - to move a value from the alt stack to the main stack

Impact on performance should be relatively small - maybe up to 5% increase in proof generation time. Proof size should remain un-affected for programs which don't make use of the alt stack.

[bobbinth]

Found a better way to implement RAM. Alt stack is no longer needed.