Update version numbers for 2.1
Remove deprecated features
Reuse "simple" object for "Implicit" in driver implementations.

"Implicit" contains a subset of "simple", so we can just cast the
same object to the desired type instead of creating a separate one.
Prevent "no-symbol does not have an owner" when using MappedTo.

Fixes issue #590. Test in RelationalMapperTest.mappedToMacroCompilerBug
(compile-time only; the test is not executed)