[foreign] RFR: tighten binder code generation

[Maurizio Cimadamore]

Hi,
this patch fixes some issues with the binder code generation:

1) In HeaderImplGenerator, the invocation of native function MH occurs 
with 'invoke' instead of 'invokeExact'

2) In StructImplGenerator, we use Pointer's boxy API to do struct 
access, when we can more simply use LayoutType's MH (which is known at 
codegen time) and do an invokeExact on it

3) remove unused box/unbox code generation helpers

Webrev:

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

Maurizio