Add list-like functions to mutable vectors

[is7s]

I find it quite annoying that list-like functions (folds, map, sum etc.) are missing for mutable vectors. Either I have to implement them myself or freeze the vector to be able to use the immutable versions. Is there a reason for the absence of these functions from the mutable vector modules?

[rl]

I completely agree but never find the time to do this. This is tracked by #11.