[foreign] RFR 8215289: Cleanup code post SystemABI integration

[Maurizio Cimadamore]

Hi,
this is a followup to the SystemABI work; the goal is to clean up some 
of the code, as I found some minor issues.

The most important issue is that there is a bug in the shortcircuiting 
logic - e.g. when doing a downcall on an upcall stub. In these cases, we 
return the method handle cached in the upcall handler, but this method 
handle doesn't always have the same signature as the one that has been 
requested by the client. The most striking case is 
UniversalUpcallHandler, whose method handle is always of type (Object[]).

Since we also uncovered another issue with this logic during the review 
process (there are questions as to whether the MH can be safely recycled 
if the new native method type is different from the old one), I decided 
to simplify the code and remove the logic altogether. If performances 
indicate that this is an issue, we can work out a story; I did some JMH 
benchamarks and found no evidence that this is the case right now (but 
we can revise if needed).

I also did a bunch of other changes:

* consolidated NativeMethodType; the class is now just an aggregate of 
layout types; the constructor that takes a Method and a Function has 
been moved into Utils, after all this is a binder-specific functionality 
(as the binder happens to know how to go from an interface/function to a 
NMT). I also added accessors in the spirit of j.l.i.MethodType to get 
return and parameter types.

* I also added javadoc to all public members of NMT

* I changed all invokers to use NMT where possible, instead of arrays of 
LayoutType

* I reverted changes in order of import in j.l.ClassLoader; these were 
pushed by mistake (probably as a consequence of some automatic IDE 
reordering). The idea is that we should try not to have deliberate 
difference with upstream, as these will cause merge failures (and 
imports is the #1 reason as to why Java code fails to merge :-))


Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8215289/

Maurizio