Replace OpenGL vector swizzling syntax with tuple indexing syntax

Now that we have tuple indexing expressions (e.g., `t.1` to get the second element), we can get rid of the very nasty hack that is the `swizzle` intrinsic. For example, `swizzle(v, "x")` becomes `v.0`.

The drawbacks are that:
- This removes the "letter" syntax for indexing, but that can get unpleasant anyway.
- We'd remove the ability to swizzle multiple elements of a vector (e.g., `v.yx`), but that's not essential.