Rolled back the validation rules a bit in :class:`.Enum` to allow
unknown string values to pass through, unless the flag
``validate_string=True`` is passed to the Enum; any other kind of object is
still of course rejected.  While the immediate use
is to allow comparisons to enums with LIKE, the fact that this
use exists indicates there may be more unknown-string-comparsion use
cases than we expected, which hints that perhaps there are some
unknown string-INSERT cases too.

Change-Id: I7d1d79b374a7d47966d410998f77cd19294ab7b0
Fixes: #3725

fixes #3095, #3292
- reorganize enum constructor to again work with the MySQL
ENUM type
- add a new create_constraint flag to Enum to complement that of
Boolean
- reinstate the CHECK constraint tests for enum, these already
fail /skip against the MySQL backend
- simplify lookup rules in Enum, have them apply to all varieties
of Enum equally