Add support for releasing the runtime lock in generated stubs

[yallop]

Fixes #241.

This adds an extra value, Cstubs.unlocked to the interface:

val unlocked : concurrency_policy
(** Generate code that releases the runtime lock during C calls. *)

Passing ~concurrency_policy:Cstubs.unlocked to the write_c and write_ml functions results in generated code that releases the runtime lock when calling C functions.