Allow alternative "by"-clause position in grammar.

In addition to the existing by-clause syntax:

  sum(<expression>) by (<labels>) [keeping_extra]

...this allows the following new syntax:

  sum by (<labels>) [keeping_extra] (<expression>)

Both orderings may be used in a single expression. It is up to the users
to establish guidelines around their usage.

Change-Id: Iba10c9cc5fb6ac62edfcf246d281473e82467992