Add mapMaybe and catMaybes

[akio]

I often want to use a vector analogue of mapMaybe from Data.Maybe. Working around it is typically awkward because it requires a use of fromJust.

I'll attach a patch that implements mapMaybe and a related function, catMaybes.

[akio]

Implements mapMaybe and catMaybes

[dleuschner]

Are there any reasons not to include this? I've just built a custom version of Vector to be able to use it but it's always a bit painful to use custom patched versions that are not released on hackage.