Convert ProjectOp and UnwindOp to map/reduce ops.

This is done via the new `WorkflowOp.finalize` function that performs
potentially-irreversible changes to the workflow prior to converting to
`WorkflowTask`. (Fixes #348)

A lot of other changes were made to support this:

* everywhere we use `Js.Expr` is now parameterized so we can specify the
  base of the expression, rather than abusing `this` all the time;
* `ExprOp`s can now _sometimes_ be converted to JS (fixing part of #272);
* map/reduce ops no longer change the WorkflowBuilder `base` to
  `ExprVar` (fixes #326);
* `FoldLeftOp` now handles patching each of its sources when necessary,
  rather than expecting the creator to do it (projecting into `value`
  for the first one, and potentially adding a ReduceOp on the others);
* `WorkflowOp.crush` tracks the `base` so it can patch operations that
  follow a `MapReduceTask` (corollary to the fix to #326), which also
  gives us some info that is useful for #287; and
* `coalesce` now simplifies some `FlatMapOp`s, eliminating ones with a
  single source and flattening nested ones when possible.

`finalize` is also a step toward #227, as that largely implies building
a `Stream` that uses different combinations of `finalize`’s
optimizations.

The one bit of nastiness (there’s always one, isn’t there?) is that I
created a `private FoldLeftOp0` that is generated by `finalize` to
indicate that the `FoldLeftOp` has already been processed. Ideally this
could be noticed in `finalize`, but my attempts were not fruitful.
Ouch. Could we add some tests in WorkflowBuilderSpec or WorkflowOpSpec and get rid of some of these? I'm surprised not to see any new tests there, although maybe your other ticket came out of this pain?