Clarify the type of Fn's jvp field

[samestep]

As of #105, the Fn type exposes a jvp field which is currently only meant to be used for setting a custom JVP on a function; to actually use the JVP, one is meant to instead call the jvp function exported by Rose. This can be confusing, since if you have f: Fn then it can seem quite natural to call f.jvp since TypeScript says it's defined; but right now that just gives undefined. Probably the right way to deal with this is to remove the jvp field from the Fn type itself since that type doesn't have enough information to say what kind of function it is, and instead put it directly into the types of things like fn and opaque that return Fns.