WIP: fixes for implicit operators and subqueries

[David Bremner]

This obsoletes the patches discussed at

     id:871rx38q8y.fsf at tethera.net

and

     id:20190821114126.23540-2-david at tethera.net

I think I still have a spelling error in the commit message that Tomi
caught, and I need to update notmuch-search-terms(7). Before I do
that, please have a look at the new tests and see if the expected
behaviour in the tests makes sense.

We still won't have working subqueries for boolean fields,
reasons discussed in

    https://trac.xapian.org/ticket/795
    
field:"(foo bar)" does not currently work for e.g. tag: or id:

There is also the unfortunate aspect that non-regex supporting fields
like "to" support Xapian to:(david tethera), but regex supporting
fields need from:"(david tethera)". It's not obvious how to fix this
without radically changing the regex syntax (e.g. to use distinct
prefixes rather than //), and I think that would annoy more people
than the quirky subquery quoting.