first working example of doors api

single-process only, with some flickering at startup, but happily flashes
the OK led for one second every other second
slightly improved crate swap

door lookup doesn't work yet, though.
door lookup now works, at least for a single door
initial try at a timer Door

still doesn't feel quite right waiting during "close", but at least it works.
removed the old "wait" code
added some screen stuff. not working, though.
added draft versions of screen handling in c
removed old references to asm screen stuff in main
referred to new screen header file
refactored out a wait helper in main, aligned framebuffer
mailbox read and write don't hang any more
added pitch 0 check, fb init should now be complete
sidetrack, implemented morse code flashing for debug use
minor, brought morse code relative duartions inline with standards
minor, typo in previous commit, and slight speedup
calls brian widdas framebuffer init code without locking
fb init works enough that I can fill screen with red pixels. \n\na bit slow, though
basic pixel writing is now reliable
removed some excess commented code
replaced missing function which had got lost in the move