par-transformers: split out safe state transformations

[rrnewton]

We originally talked about combinators for zooming in/out on data structures, kind of like lenses and zippers.

The specific case we have is morphToVec21 which acts on the left half of the state, (X,_), and performs the following transform:

• Split array A with into a tuple (A1,A2), containing non-overlapping segments of A

But we need to build up a composable series of these transforms! First, both the lens-like focusing action, and then the type-changing (but alias-freedom-preserving) transform.

cc @osa1

[rrnewton]

@DreamLinuxer, when @osa1 was last hacking on this lib, our plan was to include "mergesort" as part of the 2.0 release, and make sure everything related to it was safe and final.

But other stuff (namely the rest of par-transformers) could be left for the next release.