[plt-scheme] Foreign extensions

For whoever needed it: I finally added the ability to define
sub-structs to the foreign module.  They correspond to a struct that
contains a super-struct as its first element:

  typedef struct {
    SuperStruct s;
    ...more fields...
  };

Structs that contain the same slots of a super-struct are a little
trickier but doable.  Mail if you know of cases where this is needed.

There are a few other additions: you can define a cpointer type that
will permit NULLs, and to use unsafe features of the module you have
to use the `unsafe!' macro.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!