GFFI needs user-module injectable structs

[GoogleCodeExporter]

We currently only support structs in GFFI which are defined at GPSEE compile 
time.  This is fine our POSIX-compatibility goal, but is a poor UX for users 
wanting to use structs for third-party libraries.

The existing code works by identifying the following characteristics of each 
struct at compile-time:
 - Name of struct
 - Name of each struct field
 - Datatype of each struct field
 - Size of each struct field
 - Offset of each struct field

Once this information is known, GFFI can automatically reflect the entire 
struct into a JS object.

I would like a nice API to update this internal data structure, so that native 
modules built *outside of* of GPSEE (e.g. code in surelynx/gpsee-modules) can 
declare new structs, efficiently.

Since we can still use the C compiler to help us figure out offsetOf(), etc, we 
can still guarantee perfect compatibility across platforms, unlike solutions 
which rely on JS-runtime struct injection.

It should be possible to basically re-use the struct declaration macros, etc, 
in GFFI in another module and have everything "just work" to add new GFFI 
functionality after a given module is loaded.  Hopefully we can avoid 
macro-copying, I would like to just #include them, probably via information in 
outside.mk.

Original issue reported on code.google.com by wes@page.ca on 5 Sep 2012 at 3:37