add swap method to the Map trait

[thestinger]

Like insert, but returning the value that was replaced if applicable.

[14427]

A function along the lines of fn update(&mut self, key: K, f: fn(V)->V) to mutate a value in place would also be helpful

[thestinger]

@14427: #5528 adds a find_mut method which is quite similar

[alexcrichton]

Closed in #6236