Add namespace contextualisation for whole expressions

[gallais]

At the moment we can only write List.filter p (List.map f xs) but it would
be nice to be able to write List.(filter p (map f xs)) and have it mean the
same thing (given that none of p, f, and xs are in namespace List).

I wanted to do that but somehow forgot about it.

[pigworker]

Right.

[pigworker]

Hilariously, also

List.; filter p (map f xs)