type-specific wrapping and unwrapping of scheme values

type-specific wrapping and unwrapping of scheme values

* figl/runtime.scm (foreign-trampoline):
  (define-foreign-procedure):
  (define-foreign-type):
  (define-simple-foreign-type): Provide for a way for types to wrap and
  unwrap foreign values.

* figl/glx/types.scm:
* figl/glu/types.scm:
* figl/gl/types.scm: Adapt to define types with the runtime's macros.