Implement stream fusion

[superbobry]

Trivial example:

BS.map f $ BS.map g bs => BS.map (f. g) bs

[treeowl]

We can implement a map/map rule quite easily, without a full stream fusion framework. This particular rule can potentially increase definedness, since BS.map is strict and only accepts non-negative element values, but that's not necessarily a big problem. The general case of GS.map is trickier, at least as currently implemented. For example, with Data.BitSet.Word, mapping with a function producing "out of range" values can give a meaningful result, but it won't compose with its inverse. This could be managed through documentation, but it would be good to improve the out-of-range story generally.