copstr: COPy STRing module using const generic for capacity

copstr::Str wraps a fixed-size array of u8 and provides a string-like interface on top. The size is specified using a const generic argument.

The internal u8 array corresponds to UTF-8 encoded chars. All functions guarantee that the contents are valid UTF-8 and return an error if they are not. Truncation only happens at UTF-8 boundaries.

copstr is very useful when we want to add a string-like field to a struct that implements Copy but we don't want to give up this trait.

Example usage

use copstr;
use std::convert::TryFrom;

// Create an owned fixed-size string with size 6 *on the stack*:
let mut string = copstr::Str::<6>::try_from("string")?;

// Use it as a regular string:
println!("contents: {}", string);

// Replace the contents with another string that fits the size 6:
string.replace("str")?;

// Append a letter:
string.push('i')?;

// Instead of returning a potential error, we can instead use
// truncating methods:
string.replace_trunc("stringification");
assert_eq!(string.as_str(), "string");

// `copstr::Str` implements Deref<Target=str>, so all `str`
// methods are available:
let split = format!("{:?}", string.split_at(3));
assert_eq!(split, r#"("str", "ing")"#);

// We can add a `copstr` to a struct without having to give up the
// `Copy` trait:
#[derive(Clone, Copy)]
pub struct Mystruct {
    // ...
    comment: copstr::Str<10>,
}

// We can (and should) create a type alias:
type MyStr = copstr::Str::<4>;

// We can create `copstr` in const contexts:
const TEST: MyStr = MyStr::new_const("TEST");

When using a const context, strings that don't fit generate a compilation error. For instance, the following doesn't compile:

const TEST_BAD: copstr::Str<3> = copstr::Str::<3>::new_const("TEST");