`bytes`: BLOCKED) Make `const fn`s when `const_for` is stable.

[github-actions]

On 2022-05-28 @recmo wrote in 1ec2e50 “Recognize new tags”:

BLOCKED) Make const fns when const_for is stable.

// OPT: Use u64::from_{be/le}_bytes() to work 8 bytes at a time.
// FEATURE: (BLOCKED) Make `const fn`s when `const_for` is stable.

use crate::{
    utils::{trim_end_slice, trim_end_vec},
    Uint,
};

From src/bytes.rs:2