[jnigen] Generic support (https://github.com/dart-lang/jnigen/issues/136)

Added support for Generics – Closed https://github.com/dart-lang/jnigen/issues/66, Closed https://github.com/dart-lang/jnigen/issues/73 

* TypeClasses are now public and have `fromRef`
* Casting is possible between different types.
* Nested generic classes are also supported.