v1.0.0-M7

Changelog:

• fix a bug that occurred in some cases when using withConstructorPartial (fixed in #508)
• add withContructorEither in PartialTransformers to make it easier to work with smart constructors based on Either[String, Type] (done in #509)

This pre-release is the fifth of the planned series of internal cleanups. The series is intended to make sure that when 1.0.0 is finally released it will not have any serious (known) issues that could be only addressed by breaking the backward compatibility. We will be really glad if you tested it, and let us know about any such issues while they are still easy to fix.