Rename VM memory methods.

alloc -> alloc_block
realloc -> realloc_block
free -> free_block

This avoids having them clash, obviously, with realloc and free and
makes it clearer which one is in use.
Remove commented out code.
Chop down MOD case.
Correct documentation for PEEK case.
Use const pointers in PEEK.
Add initial bounds checking for MEMMOVE.
Add a block bounds checking function.
Add bounds checking to PEEK/POKE.

Uses check_block_bounds.
Trim MEMMOVE instruction with check_block_bounds.

Reduces the need to do as much staring a the code to see if the src/dst
variable prefixes are right. Not perfect, but it'll have to do.