BETWEEN, a more mature approach

This is a work in progress.

`RANGE` is now compiled to construct an array of values.

`Between` is now handled as follows in the planner phases:
1. an `ExprOp` is constructed, but not yet used (not the correct one yet; there will be some work to represent all the expressions we support as exprOps, but I believe everything is in place to do this work). This expression will eventually be used to translate portions of the expression to JS for use with `$where`.
2. a `Selector.And` instance is constructed which is actually used.
3. the pipeline phase has a case for `Between` so as not to fail, but does nothing interesting with it. Not sure what we actually need there, if anything.
The syntax is `x BETWEEN min AND max`, and it's parsed as `Between(x, Range(min, max))`. It's really the `Range` that's causing the problem. It's recently occurred to me that just parsing it as `Between(x, min, max)` would eliminate a lot of these complexities.

I don't want a more powerful paramorphism, I want a _less_ powerful one. But I'll stick with `scanPara0`, as you say.