[foreign] RFR: typeful upcalls do not work

[Maurizio Cimadamore]

Hi,
I realized that UpcallHandler doesn't cope well with typeful callbaks - 
e.g. when a callback features generic types (e.g. Pointer<Integer>) in 
argument position. This is due to the fact that UpcallHandler throws 
away info asscoaited with the reflective method (which has generic 
signatures available) and uses MethodType instead.

This patch corrects that. In doing so, I also took the opportunity to 
remove some obsolete constraints from the impl:

* callback must have @FunctionalInterface in there

* callback method should be called 'fn'

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/typed_upcall/

Cheers
Maurizio