BUGFIX:
 - inverse-time feedrate mode now works properly with helical and
circular movement too. Also, this helps kinematic code in the future
since we're calculating distance to go.
SNAPSHOT:
 - relayed radComp all the way to enqueue_move(), thus allowing
compensation to occur at the logical spot, in the aforementioned
function.
UPDATE:
 - radius compensation is in! 'Nuff said :-) Probably needs a little
polishing in the weeks to come, but boy was it hard to get right.