Rework how global memory works

[depombo]

Global memory doesn't have a good way of representing variable memory types adding a lot of corner case handling for strings, which are currently "blessed" as the only variable-sized type allowed in global memory. Making all types representable in Global memory would unify it with "normal" memory in the AVM and allow the special case opcodes and conditionals in the AVM to go away, reducing possible bugs dramatically and providing some perf boost to the AVM by making memory manipulation less "branchy."