My proposed changes in the tag query parser are motivated by the need and/or
desire to do the following. (The example strings work with the new parser.)

1. Combine and modify tag queries programmatically.

   The leading case is that a function is given a tag query string and
   needs to *exclude* lines matching that query. To do this, we can
   transform query strings like so:

     "foo+bar+zap/TODO"  -->  "-(foo+bar+zap/TODO)"
     "foo|bar|zap"       -->  "-(foo|bar|zap)"

   The key is that we want to do this programatically while still using
   the mapping or agenda search command.^* I use this a lot in my GTD
   layer for org; other combinations and transformations come up as well.

2. Write complex queries as simply as possible (i.e., using parens).

   Parentheses aren't always necessary, but they can make things nicer.

   "(xyz|{^a}-abc) & LEVEL > 1"  versus "xyz&LEVEL>1|{^a}-abc&LEVEL>1"

3. Make *fast* heading and priority searches

   That information is *already matched* in the current code but access
   is not given (or is slow in the case of PRIORITY).

   "LEVEL == 2 & HEADING <> {<.*>} & PRIORITY <> \"A\" "

4. Include braces in regular expression matches.

   "+{abc\\{{3,7\\}}}"             -> regex "abc\\{3,7\\}"
   "{[A-Z]+\\S-+{{template}}.*$}"  -> regex "[A-Z]+\\S-+{template}.*$"

   Because \ escapes are used so heavily in regexex and because strings
   require doubling them, using additional \'s would be messy,
   ambiguous, and hard to read.

   Instead, exploit that we only need to protect {}'s by *doubling*
   them: {{ -> { and }} -> }. This is simple, readable, fast, and parity
   makes correctness clear at a glance.^**

5. Allow spaces in query strings for readability.

   Not a big deal, but easy. See the above examples

6. Get helpful error messages at parse time when there is a problem.


^* It is of course possible to create a matcher from the string
   and do the search directly with lower level functions, but
   that ends up being a clunky solution.

^** The doubling strategy is also familiar from the doubling of \'s
    in quoted strings.