WIP: fix implicit operators

[David Bremner]

I'm posting this for feedback.

I did all the tests that were easy, but things like threads and
attachments will take more effort to test.

for probabilistic prefixes, we are basically stuck with AND. For
boolean prefixes, we have the choice.  Generally I tried to follow the
Xapian convention that if a field/prefix is "exclusive" (one per
message), then we group with OR.  Regexp fields like mid:// are tricky
since even if the underlying data is exclusive it makes sense to use
multiple instances to match one piece of data, e.g.

mid:/david/ mid:/tethera/

should probably behave roughly like

mid:/david.*tethera/