Restrict TUPLE_INDEX to only allow decimal digits

[matthewhughes934]

I was rather confused after reading the current definition and trying out some code:

struct Foo(u32);

fn main() {
    let f = Foo(1);
    let _ = f.0;    // ok
    let _ = f.0_0   // DEC_LTIERAL: error[E0609]: no field `0_0` on type `Foo`
    let _ = f.0b0;  // BIN_LITERAL: error[E0609]: no field `0b0` on type `Foo`
    let _ = f.0o0;  // OCT_LTERAL:  error[E0609]: no field `0o0` on type `Foo`
    let _ = f.0x0;  // HEX_LITERAL: error[E0609]: no field `0x0` on type `Foo`
}

[matthewhughes934]

I thought it was about as much effort to raise a PR with this change as to raise an issue explaining the problem. Though I'm aware the contributing guide asks for opening issues in these cases, so sorry if I've jumped the gun.