Wrap `JMethodIDPtr` and `JFieldIDPtr` into high level classes

These classes can be called `JMethod` and `JField`. `JMethod` will have methods to call it with arguments and `JField` will have methods to get/set it.

These two classes can be used by `package:jni` users instead of using the `Jni.env` and `Jni.accessors` primitives and by `package:jnigen`'s generated bindings.