Add heuristics for and/or queries

Created by: rvantonder

A number of parser heuristics make regexp and other search modes more usable in the presence of and/or queries. It performs a similar role to "autofix" that we have for simple queries, but in a more principled and tractable way.

Part of this work was initially unplanned but became increasingly important and implemented for 3.15 as part of #8567 (closed). This includes work involved in implementing a mapper interface to do various transformations.