Optimise bounds checking for memory and tables

[SimonJF]

Currently, memory (and soon tables) require explicit bounds checks to ensure safety. This incurs an additional read, and certainly additional branches.

One technique to reduce this would be to use mmap to ensure that out-of-bounds accesses cause a segmentation fault, and then handle the segmentation fault by trapping. Thus, the assumption is that the memory access will succeed, but that there's a guarantee that a segfault will be raised on out-of-bounds accesses.