Reduce UWORD to unsigned char

[roconnor-blockstream]

Double check this, but the memory allocation of many bit sized frames ends up bloated due to the large UWORD size. Better just to make it 8-bit when possible.

[roconnor-blockstream]

The number of frames is limited to less that CELLS_MAX, which is currently 5 Mebi.
Each frame is at least one UWORD, which tends to be 8 bytes in size.
A frameItem, contains the frame location and cursor location, is 16 bytes, though could probably be reduced to 8 bytes.

So the total allocation of the cells and the frame stack is at most CELLS_MAX×(8+16) = 120 Mebibytes, which probably isn't so bad. Reducing the frameItem size would be slightly more impactful than reducing the UWORD size, and should probably be pursued first, as part of a broader memory reduction refactoring (see #125).

The current UWORD size means moving frames has very good alignment, and maybe that is helpful?

I'm going to close this for now as I consider the current allocation acceptable.