Takes a proxy instance and a map of strings (which must correspond to methods of the proxy superclass/superinterfaces) to fns (which must take arguments matching the corresponding method, plus an additional (explicit) first arg corresponding to this, and updates (via assoc) the proxy's fn map. nil can be passed instead of a fn, in which case the corresponding method will revert to the default behavior. Note that this function can be used to update the behavior of an existing instance without changing its identity. Returns the proxy.