Add support for 'const' and 'volatile'

[yallop]

Add two new operations on types:

val const : 'a typ -> 'a typ
(** [const t] const-qualifies the type [t].  At present the only
    effect is that the type is marked 'const' in generated code. *)

val volatile : 'a typ -> 'a typ
(** [volatile t] volatile-qualifies the type [t].  At present the only
    effect is that the type is marked 'volatile' in generated code. *)