Ravel / & / Fill

New operator on CoreBag
* & (intersection) operator on CoreBag - is part of Set interface

Fill improvements
* new fill function - quite a bit quicker in most instances
* deprecating 2D fill - not used particularly, doesn't work with new fill

Ravel: a function to take arbitary lambdas and return chunks ala xyzzy
Not terribly quick, could possibly do with implementing memoisation on bags.
Was called plugh at one point, hence name of branch.