This implements `INLINESTATS`. Most of the heavy lifting is done by
`LOOKUP`, with this change mostly adding a new abstraction to logical
plans, and interface I'm calling `Phased`. Implementing this interface
allows a logical plan node to cut the query into phases. `INLINESTATS`
implements it by asking for a "first phase" that's the same query, up to
`INLINESTATS`, but with `INLINESTATS` replaced with `STATS`. The next
phase replaces the `INLINESTATS` with a `LOOKUP` on the results of the
first phase.
So, this query:
```
FROM foo
| EVAL bar = a * b
| INLINESTATS m = MAX(bar) BY b
| WHERE m = bar
| LIMIT 1
```
gets split into
```
FROM foo
| EVAL bar = a * b
| STATS m = MAX(bar) BY b
```
followed by
```
FROM foo
| EVAL bar = a * b
| LOOKUP (results of m = MAX(bar) BY b) ON b
| WHERE m = bar
| LIMIT 1
```
