Do not allow consequtive zeroes as a name of tuple struct fields

[matklad]

The following compiles, but I think it shouldn't?

struct S(i32); 
fn lol(s: S) { s.00000; }

[petrochenkov]

The field name is converted to a number too eagerly.

This doesn't happen in struct expressions and patterns.

struct S(i32); 

fn main() {
    let s = S { 0000: 10 }; // ERROR struct `S` has no field named `0000`
    let S { 0000: x } = s; // ERROR struct `S` does not have a field named `0000`
}