Use more concrete types for NamedFunction constants in the code

On 1/21/15 10:30 PM, John Rose wrote:
> in Invokers.java, I think that checkCustomized should take an Object and not a MethodHandle
> exactly like getCallSiteTarget takes an Object and not a CallSite.

The use of erased types (any ref => Object) in the MH runtime is an artifact of bootstrapping difficulties, early in the project. I hope it is not necessary any more.