major refactoring

- fix bug when game restarts and board doesn't get cleared
- optimize by using immediate opcodes and fallthrough subroutines
- lint with uxnlin
- format with uxnfor
- store x and y coordinates of the cursor in different bytes
(i thought this was a cool memory hack, but since the variables
are located in zero page this doesn't save memory, on the contrary,
it adds unnecessary complexity to manage packing and unpacking
coordinates from nibbles)
- remove loops from check-win
- remove nested comments
- avoid redoing some work when game restarts (restart code
now jumps to code that is necessary to only reset game state and
clear the board)

readme: correct prerequisites

apply optimizations

add lint to makefile

makefile: allow uxnasm to be used as an assembler

init