Adds type hints to support static analysis with Psalm

Add type hints for static linting with Psalm (#10)

This PR adds type hints and docblocks to support static analysis of your custom mapper classes with Psalm.

Most notably, `SourceAdapter`, `DestinationAdapter` and `Mapper` implementations can make use of templated types in Psalm, which is a great help due to the generic (`object` based) approach in this package.

The types are:

* `Ts` type returned by the `SourceAdapter`
* `Tr` type returned by `DestinationAdapter::getObjectsOrderedById()`, `r` for "reading"
* `Tw` type used by the `DestinationAdapter` when writing back to the destination system.