We like static assertions...
Tweak DTD completion.
Monkey now can have multiple dtds outstanding & knows how to cancel
old ones to free up space.
It's safe to disable/enable interrupts from our interrupt handlers...
Getting monkey SSP out going via DMA.
Fixup the monkey pointer calculations...
It is only valid to wrap insert_pos when we are about to set the limit.
Freeing space can invalidate everything.
Simplify headroom calculation.
Tidy up the lcd code a bit.