Give the user control over the lifetime of closures passed to C.
Rather than passing OCaml function values to C we now pass an integer
identifier that resolves to a callable object so long as the original
closure is still live.
Merge commit '8ae5a3905854aeff0ddfeeee4ed66aadc3a8dc4e' into function-lifetimes-clean

Conflicts:
	_oasis
	_tags
	src/ctypes.ml
	tests/clib/test_functions.c
Merge pull request #37 from yallop/function-lifetime

Give the user control over the lifetime of closures passed to C