Flagged heads as "cannot be passed through".
Fixed portal frame height and added a few new blocks to centralTopLimit
Added Block and id/data overloads to canPassThrough, centralTopLimit and isNaturalTerrainBlock.
Made everything that uses canPassThrough use data values.
Flagged all snow layers except single layers (data values 0 and 8) as "cannot be passed through".
Optimized EditSession.countBlocks.

Also fixed the questionable semantics for that anonymous class' "contains" method.
Adjusted shouldPlaceLast.
Made /ascend and /descend use centralTopLimit.
Fixed centralTopLimit for trap doors and fence gates.
Fixed centralTopLimit for cake blocks with data values 6..15.