add backend agnostic UUID datatype

Added new backend-agnostic :class:`_types.Uuid` datatype generalized from
the PostgreSQL dialects to now be a core type, as well as migrated
:class:`_types.UUID` from the PostgreSQL dialect. Thanks to Trevor Gross
for the help on this.

also includes:

* corrects some missing behaviors in the suite literal fixtures
  test where row round trips weren't being correctly asserted.
* fixes some of the ISO literal date rendering added in
  952383f9ee0 for #5052 to truncate datetime strings for date/time
  datatypes in the same way that drivers typically do for bound
  parameters; this was not working fully and wasn't caught by the
  broken test fixture

Fixes: #7212
Change-Id: I981ac6d34d278c18281c144430a528764c241b04

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