Be more clever about abbreviating variable lists

[fingolfin]

Consider this:

julia> R = @free_associative_algebra(GF(2), [:x, :y, :u, :v, :t, :s])
Free associative algebra on 6 indeterminates x, y, u, v, ..., s
  over finite field F_2

So the variable list was "abbreviated" from x, y, u, v, t, s to x, y, u, v, ..., s which is actually two characters longer. It would be nice if we didn't do that.