Switch to numeric fields in struct expressions

[dtolnay]

This syntax was stabilized in Rust 1.19 with the intention of simplifying code generators that instantiate tuple structs. Tracking issue: rust-lang/rust#35626.

struct S(A, B, C);

let _ = S { 0: A, 1: B, 2: C };

When bumping our minimum required compiler version we will want to run through our Deserialize derive and see where this syntax would allow us to share more code between the struct and tuple struct code paths.