Flatten operator precedence for comparison operators

Flatten operator precedence for comparison operators

The operator precedence for all comparison operators such as LIKE, IS,
IN, MATCH, equals, greater than, less than, etc. has all been merged
into one level, so that expressions which make use of these against
each other will produce parentheses between them.   This suits the
stated operator precedence of databases like Oracle, MySQL and others
which place all of these operators as equal precedence, as well as
Postgresql as of 9.5 which has also flattened its operator precendence.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #3999
Change-Id: I3f3d5124a64af0d376361cdf15a97e2e703be56f
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/367