perf: reduce allocation for parsing

[h-a-n-a]

Summary

1. Prefer passing a copy than a pointer

struct DependencyId(u32); // <- 4 bytes

fn foo(d: &DependencyId); // <- usize
⬇️
fn foo(d: DependencyId); // <- 4 bytes

Pointers in rust are usize. On a 32 bit target, this is 4 bytes and on a 64 bit target, this is 8 bytes.
A tuple of a u32 is 4 bytes.

2. Shadowed Display of Identifier with to_string

Like how other structs are shadowed in the standard library for performance, we shadowed Identifier to gain performance of calling to_string instead of directly calling display.
Calling display basically means calling format!("{}", xxx). This creates additional allocation for formatters.

See also: rust-lang/rust#32586.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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