This only allows for indexing with a constant value (e.g. x[0]).
While slices would be possible to implement, it's not clear how to
preserve type inference here. The current typing rule is:
Γ ⊢ x : τ Γ ⊢ a : Int Γ ⊢ b : Int
------------------------------------
Γ ⊢ x[a:b] : τ
However, tuples would require a different typing rule, and so we'd need
to defer type inference if τ is a tyvar. I'm not confident that this
won't change behaviour, so we leave as-is for now.
Signed-off-by: Jonathan Coates <jonathan.coates@oxionics.com>