improvements to option module

[thestinger]

I didn't add an equivalent free function for map_consume_default, mutate and mutate_default because as far as I can tell the only purpose of the free function versions is backwards compatibility.

The mutate methods are based on a need I have in my balanced tree implementation, which arises from doing it entirely without Copy/Clone. It factors out variations of the following pattern which occurs many times:

if foo.inner_option.is_some() {
let inner = swap_unwrap(&mut foo.inner_option);
inner = transform(inner);
foo.inner_option = Some(inner);
}

[catamorphism]

Thanks!