Change engine VM memory layout as follows:

Change engine VM memory layout as follows:

0 to 0x7FFFFFFF: reserved.
0x80000000 to 0x80000000 + num_globals - 1: global pointers.
0x80000000 + num_globals to 0x80000000 + num_globals + num_entities * num_entityfields - 1: entity field pointers.

Note that there are no instructions yet to _make_ global pointers, or to
load values from them.

This is the beginning of implementing the pointer instruction set.