Collision detection stable, but buggy

stable as it ever was - still trouble with corners and landing higher
than you started.
Collision detection functional, but non-optimal

Collision detection for bottom, left, right is now working.
If lower-hardness object (e.g. player) hits a harder object above it
(e.g. a ceiling) then untested.
Current code is far from optimal, should be vastly neatened before
proceeding.