Remove all special handling for products of size 1.

The new query compiler can deal perfectly well with nested products,
so there is no more need for ugly hacks that fail for products of
size 1.

We can now also add a Shape for Tuple1 for completeness.

Fixes issue #658. Tests in JdbcMapperTest.testSingleElement.
Merge pull request #661 from slick/tmp/one-off

Remove all special handling for products of size 1.