[RFC PATCH v2 0/8] Custom query parser, date search, folder search, and more

[Austin Clements]

This is version 2 of the custom query parser.  It now supports date
searches with sane syntax, folder searches (without any additions or
changes to the database, unlike cworth's recent commit), and "tag:*"
and "-tag:*" queries for finding tagged and untagged messages.  I used
these features to guide changes to the original design and to validate
the approach.  This is still RFC, but it's much less raw now.

In addition to the new features, the core query parser has a bunch of
cleanups and changes, including completely redone NEAR and ADJ
operators that now behave essentially the same as they do in Xapian's
query parser.  I also split the implementation of these out into a
separate patch for ease of review.

There's a notable lack of tests in this current series.  I do have a
pile of tests for the lexer, parser, and generator, but the
infrastructure for testing them needs cleanup before I send that out.