Rename Invoker.to to Invoker.buildColl and deprecate the old name.

This will free up "to" for use in the new encoding of type constructors
into Query for Slick 2.1. We should be able to get rid of Invokers in
2.1 and do everything with the simple Executor.
Merge pull request #556 from slick/tmp/deprecate-to

Rename Invoker.to to Invoker.buildColl and deprecate the old name.