Allow custom opaque functions

[samestep]

This PR adds a custom function to construct opaque functions that simply call an arbitrary JavaScript function. To achieve this, the rose::FuncNode trait has been split into a rose::Refs trait and a rose::Node enum, the latter of which has two cases for the previously-existing Transparent functions and the new Opaque functions.

This initial implementation only supports opaque functions with very simple signatures: a small number of F64 parameters, and an F64 return value.