[WIP] Sequence

The original intent was:

1. Implement `sequence<R: ReducibleType>(reducible: R) -> SequenceOf<R.Element>` so that arbitrary reducibles can easily be iterated using `for`/`in`.
2. Remove `ReducibleOf`’s conformance to `SequenceType`, resolving the ambiguity described in #18.

This ballooned somewhat when working on `concat`—some tests made it obvious that something was amiss, and in fact it turns out that `flattenMap` is traversing the nested reducibles out of order. (This has not yet been resolved.)

Debugging this turns out to be quite difficult.

Fixes #18.