intmap: add iterator-by-callback.

intmap: add iterator-by-callback.

It's significantly faster because it assumes no deletion:

10000000,critbit iteration (nsec),316
10000000,critbit callback iteration (nsec),90
...
10000000,critbit consecutive iteration (nsec),308
10000000,critbit consecutive callback iteration (nsec),78

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>