[foreign] RFR: switch to toplevel annotations

[Maurizio Cimadamore]

Hi,
this patch completes the migration towards the annotation model 
described in [1].

More specifically, @NativeStruct, now contains a full layout 
description, with layout annotation which point to Java accessors. 
@NativeHeader contains a set of declarations of the kind 
'symbol=descriptor', where 'symbol' is the linker name of the given 
symbol, whereas descriptor can be either a function descriptor or a layout.

I did some refactoring of the binder code, which I think helped in 
making it even clearer, I hope.

Note that now the binder doesn't depend on @NativeType, nor @Offset, but 
I left those in place because we have some jextract tests depending on 
them (jextract still needs to be fixed to generate the new stuff).

In order to compute offset of fields, I added a new LayoutPaths helper 
class, which allows the binder to get a layout path given a predicate 
expression and a layout. This is useful to get a path to a given struct 
field, which then gives you info about things such as offset (and, maybe 
in the future, alignment).

Of course, many tests have to be updated; since I touched the dreaded 
UnixSystem test, it'd be better to give it a try on MacOS.

Webrev:

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

Cheers
Maurizio

[1] - http://cr.openjdk.java.net/~mcimadamore/panama/panama-binder-v3.html