use fnmut for functions on map values

[dwightguth]

This improves the fix for issue #7 in order to allow users to write functions over map values that can record state. This is useful if you want to report an error if you are unioning two maps and discover conflicting keys. You can write something like:

let mut error = false;
let union = m1.clone().union_with(m2.clone(), |v1, _| { error = true; v1 });
if error {
  // do something
}

This was not possible previously because of the constraint Fn on the function argument to union_with instead of FnMut. Note that because Fn implements FnMut, this is a strict superset of the functionality accepted previously and should be fully backwards compatible to the best of my understanding.

[bodil]

Huh, I was sure I'd done this already, it must have gotten lost somewhere... Thanks, merging!