Kernels should be able to grow or shrink input

[eholk]

If effect, we need an efficient way to do concatMap.

One way I've been considering is changing out underlying vector data structure to be a rope. This should support efficient concatenation, but may also have an unacceptable performance penalty for straightforward iteration cases.

[eholk]

It's also possible to implement ropes as an ADT and some associated libraries, perhaps with a new kernel macro to enable easy kerneling over ropes. To make this really useful, we'll want polymorphism for ADTs.