Access to video manipulation from Forth

[fgc]

Defvars for curvid, curline (maybe a defconst for vidram too) would be nice to have in order to implement page (clear screen) and at-xy (output at a particular place in the terminal). In particular "page" seems a good candidate for implementation via the forth assembler.

[fgc]

Seems simple enough that I could try and provide a pull request myself