implement regex::Replacer for String, &String, Cow<'a, str>, &Cow<'a, str>

[nooberfsh]

It is inconvenient to get &str in some cases, for example:

extern crate regex; // 1.4.2
extern crate chrono; // 0.4.19

fn main() {
    let input = "today is time";
    let today = chrono::Local::now();
    let re = regex::Regex::new(r"time").unwrap();
    let s = re.replace(input, &*format!("{}", today)); // <-- I have to use &*format(...)
    println!("{}", s);
}

[BurntSushi]

Thanks! I think this LGTM, but I'd like to see two changes before merging it:

1. Update the docs on the Replacer trait. Right now, it calls out the &str impl specifically, but now we've added more. I don't think we should list out every impl, but maybe say, "implemented for &str along with other variants of string types." Or something like that.
2. Please also add the analogous trait impls in src/re_bytes.rs. It seems weird to offer these extra impls for only the &str API.

[nooberfsh]

@BurntSushi updated.

[BurntSushi]

LGTM, thanks!