Add insertion functions

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](http://stackoverflow.com/a/28665345/1477667) on StackOverflow; `insertAt` should be particularly simple.