[foreign] RFR: add binder support for bitfields

[Maurizio Cimadamore]

Hi,
this patch adds binder support for bitfields. I did a bit of cleanup on 
StructImplGenerator & RuntimeSupport - now RuntimeSupport is simply a 
factory for useful method handles that are to be injected (via CP 
patching) into the generated code. This allows code generation to 
abstract away from details such as: is the accessor for a bitfield or 
for a regular struct field? We just get the right MH and we call it with 
invokeExact.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/binder_bitfields_v2/

P.S.
jextract support to follow

Maurizio