[fixed-hash] to_string() gives shortened version instead of full

How to reproduce

here is snippet

// fixed-hash is 0.7.0 (latest)
use fixed_hash::construct_fixed_hash;
construct_fixed_hash! {struct H160(20);}

fn main() {
    let hash = H160::zero();
    // Debug formatting
    println!("{:?}", hash); // 0x0000000000000000000000000000000000000000

    // Display formatting
    println!("{}", hash); // 0x0000…0000

    // ToString
    println!("{}", hash.to_string()); // 0x0000…0000
}

Expected results

to_string() returns string representation of H160

Real results

to_string() returns shortened version

Source of problem

This occurs due to default implementation of ToString in standard library: impl<T: Display + ?Sized> ToString for T

Proposed solution

swap realizations for Debug and Display

Advantages

• More suitable implementations of formatting traits (Shortened is useful for quick eye-checking, and when Display-ing to user we usually need full hash.)
• Generates same amount of code, do not increase load to compiler

Disadvantages

• It's a breaking change

Alternative solution

add codegen for ToString, which will return full version of hash

Advantages

• Non-breaking change

Disadvantage

• Generates more code, slowing compilation a bit
• Leaves unsuitable implementation of formatting traits

Start at the construct_fixed_hash! macro and inspect the generated Debug and Display formatting implementations. Run the reproduction snippet to confirm the current shortened output and compare the compatibility implications of the proposed formatting changes. Done means deciding on an approach and ensuring to_string() returns the full H160 representation without regressing the relevant formatting behavior.