Fix WISL cell creation bug

[NatKarmios]

WISL is currently set to, when executing SetCell/SetBound/SetFreed, create the necessary heap entries if the relevant cell is missing. This makes sense when producing assertions, but causes false negatives when these actions are used directly in programs.

For example, this program succeeds, where one would expect it to fail as there is no cell at x:

{ (x == #x) }
function main(x) {
  [x] := 4;
  return null;
}
{ emp }

This change adds a alloc_if_missing argument to various functions, including WislSMemory.resolve_loc, WislSHeap.set_cell, etc. which specifies whether this behaviour should occur; it is off by default, but enabled inside WislSMemory.produce.