Added lock prefix and did some refactoring for the prefix (and added a test)

[s-prism]

• (Added a single "not unit" test)
• Added lock prefix. Refactored so that in the ast, some instructions have a field prefix, where prefix('wsize) = struct {lock: bool, wsize: int('wsize)}
• Unfortunately this refactoring is very messy / has added complexity to the typing, as our prefix struct needs to have a type parameter 'wsize, so that we can enforce all the type constraints from before