Add a `subspaces` function for finite vector spaces

[fingolfin]

... perhaps similar to what we have in e.g. GAP. I'll quote its documentation:

Subspaces( V[, k] )
Called with a finite vector space V, Subspaces returns the domain of all subspaces of V.

Called with V and a nonnegative integer k, Subspaces returns the domain of all k-dimensional subspaces of V.

Special Size (30.4-6) and Iterator (30.8-1) methods are provided for these domains.

Ignore the fancy bit about "domains", for us this could simply be an iterator.