Shrink the size of JS objects by 2 pointers, so
8 bytes on 32-bit, 16 bytes on 32-bit.

This is done by pushing pointers that are not really needed
inside ObjectImp to subclasses; and also by rearranging and bitfielding
some things inside of the RegExp constructor.

Makes the cell size (32-bit, I need configury for 64-bit!) go
56->48 bytes, with GC adjusted accordingly.

Makes things may be 1.2%-1.5% faster; this version of the patch
is after adjustment by Apple to reduce divergence...