Rectangle operation commands

[MightyPork]

filter, fill etc (vt500?)

[MightyPork]

as a pre-requisite, this will need unicode to be stored in the decoded form.

a) change the format used by the utf8 cache to numbers
b) get rid of utf8 cache and all the hacks to make it work, and expand Cell.ch to a 21-bit number. This takes roughly 3.000 extra bytes of RAM. we can probably afford this tho

Moving the high attr bits into the top nibble of the unicode number (there are 3 unused bits) would reduce the RAM expansion by 2.000 bytes

As another alternative, we could forbid the use of utf8 points over U+FFFF

[MightyPork]

on a second thought, we need only 17 bits for unicode, anything above U+1FFFF is CJK extensions and private use