New IMAP search parser/generator

Previously, the IMAP search tried to do sneaky things to optimize
execution time, but that ended up with problems when nested ()s
were used, among other, more subtle issues.  Also, the old search
wasn't even tested enough so that each term would work.

The new system transpiles the IMAP query to Javascript then runs
the compiled JS function for each message.  Should be much more
accurate (though may also be much slower).

Actually fixes the issue reported as #397, and closes #730.