Implement map in place

[japaric]

Sugar for:

for e in m.iter_mut() {
    f(e)
}

Probably via a Map trait:

trait Map<T> {
    fn map(&mut self, f: |&mut T|);
}

Although it'd be nice to just write m.map(Float::sin) or even better just m.sin() (like Python universal functions).

This can be thread parallelized in the future.