Support vector string indexing

[Dheu3]

let s = ["hello"]
let x = int2{1,0}
print s[0][1]
print s[x]

Expected that line three and four would be the same, printing 101.

[aardappel]

Yes, that generally works across N vectors, but strings are not vectors, they are their own thing :)

I suppose we could enable this, but it be highly specialized, requiring its own codegen etc, so I am fine leaving it as is.