add additional IMV UUID tests, fix pymssql case

add additional IMV UUID tests, fix pymssql case

Fixed an issue regarding the use of the :class:`.Uuid` datatype with the
:paramref:`.Uuid.as_uuid` parameter set to False, when using the pymssql
dialect. ORM-optimized INSERT statements (e.g. the "insertmanyvalues"
feature) would not correctly align primary key UUID values for bulk INSERT
statements, resulting in errors.

This change also adds a small degree of generalization to the
Uuid datatype by adding the native/non-native compilation conditional
to the base compiler.

Patch is originally part of Ib920871102b9b64f2cba9697f5cb72b6263e4ed8
which is implementing native UUID for mariadb in 2.1 only.

Change-Id: I96cbec5c0ece312b345206aa5a5db2ffcf732d41