A way to read [a] to t a

Hi,

It would be nice to have functions for converting lists to matrices and vectors. I imagine something like this:

```
class FromList t where
fromList :: [a] -> Maybe (t a)

instance FromList V4 where
...
```

, sans class and function names. Writing those functions by hand (with pattern-matching) is long and error-prone -- better have them in one place. For example, this is useful when I parse data from a file. I'm willing to provide a patch if we agree on an interface.

Finally, it's very possible there is already something to do this and I've simply overlooked it. If so, sorry for this!