post queries: refactor AST #simplify method.

Refactor the `PostQuery::AST#simplify` method to split it into three
methods: `#trim` to eliminate redundant AND and OR clauses, `#simplify`
to expand deeply nested subexpressions, and `#sort` to sort the query
into alphabetical order.

This is so we can normalize queries written by users by parsing and
rewriting them, but without expanding out nested subexpressions, which
can substantially alter the way the query is written.

test/unit/post_query_parser_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 class PostQueryParserTest < ActiveSupport::TestCase
   def assert_parse_equals(expected, input)
-    assert_equal(expected, PostQuery::Parser.parse(input).simplify.to_sexp)
+    assert_equal(expected, PostQuery::Parser.parse(input).to_cnf.to_sexp)
   end
 
   def to_infix(string)