Add insertion functions #76

I doubt these are needed too terribly often, but it might make sense to add them anyway—the usual Vector API doesn't seem to offer any way to implement them efficiently.

1. insertBeforeFirst :: (a -> Bool) -> a -> Vector a -> Vector a
2. insertAfterLast :: (a -> Bool) -> a -> Vector a -> Vector a
3. insertAt :: Int -> a -> Vector a -> Vector a

I've written two fusing versions of (essentially) insertBeforeFirst in an answer on StackOverflow; insertAt should be particularly simple.