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
```
* Enforce an invariant in our dependency checker so that logical plans never have duplicate output attribute names or ids.
* Fix ROW to not produce columns with duplicate names.
* Fix ResolveUnionTypes to not create multiple synthetic field attributes for the same union type.
* Add tests for commands using the same column name more than once.
* Update docs w.r.t. how commands behave if they are used with duplicate column names.
This copies the first line of the description of each command to just
under the syntax so that it's "in order", before the `Parameters`
section. That way if you are reading from top to bottom you see:
```
syntax
short description
parameter names and descriptions
long description
examples
```
I've also removed the `Description` section entirely if the description
was just one sentence. So in some cases that just isn't `long
description`.
