Support Scratch block: "if on edge, bounce"

[towerofnix]

Branching off of #18.

Development ongoing in #114 and #119, thanks @HanClinto and @adroitwhiz!

Original implementation in #59 by @apple502j.

[another-m00]

The offical code for this block is If on edge Bounce. It checks the distance from all sides, and uses tangent to bounce, nothing fancy.

If we have a fencing function that keeps the sprite within bounds I can easily trascribe it if you guys want it.