proposal: remove public symbols that should be private

[kardianos]

A number of consts and possibly others are public, but should be private.

Users will mostly not need to use this package directly, but there are cases where they should.

Clean up and document the usable API better.
Possibly move user visible bulk API into one sub-package, and custom types such as VARCHAR and UUID into another sub-package. Then the driver docs can be ignored (though it should still be cleaned up).