Change use constructors instead of functions

[simonbyrne]

It's a minor thing, but for Dataspaces and Datatypes, it would be nice if we could use the constructors directly instead of going through functions. e.g. Dataspace((4,4)) instead of dataspace((4,4)).

[mkitti]

Should we just turn the functions into aliases of the constructors?

[simonbyrne]

In #1104, I changed it so that Dataspace is a basic constructor, but you use dataspace(obj) to get the Dataspace of an object. I think that distinction is helpful, as it can avoid some ambiguities (e.g. getting the dataspace of a tuple)