This adds a guarantee to the API of the `replace`, `replace_all` and
`replacen` routines that, when `Cow::Borrowed` is returned, it is
guaranteed that it is equivalent to the `haystack` given.

The implementation has always matched this behavior, but this elevates
the implementation behavior to an API guarantee.

There do exists implementations where this guarantee might not be upheld
in every case. For example, if the final result were the empty string,
we could return a `Cow::Borrowed`. Similarly, if the final result were a
substring of `haystack`, then `Cow::Borrowed` could be returned in that
case too. In practice, these sorts of optimizations are tricky to do in
practice, and seem like niche corner cases that aren't important to
optimize.

Nevertheless, having this guarantee is useful because it can be used as
a signal that the original input remains unchanged. This came up in
discussions with @quicknir on Discord. Namely, in cases where one is
doing a sequence of replacements and in most cases nothing is replaced,
using a `Cow` is nice to be able to avoid copying the haystack over and
over again. But to get this to work right, you have to know whether a
`Cow::Borrowed` matches the input or not. If it doesn't, then you'd need
to transform it into an owned string. For example, this code tries to do
replacements on each of a sequence of `Cow<str>` values, where the
common case is no replacement:

```rust
use std::borrow::Cow;

use regex::Regex;

fn trim_strs(strs: &mut Vec<Cow<str>>) {
    strs
    .iter_mut()
    .for_each(|s| moo(s, &regex_replace));
}

fn moo<F: FnOnce(&str) -> Cow<str>>(c: &mut Cow<str>, f: F) {
    let result = f(&c);
    match result {
        Cow::Owned(s) => *c = Cow::Owned(s),
        Cow::Borrowed(s) => {
            *c = Cow::Borrowed(s);
        }
    }
}

fn regex_replace(s: &str) -> Cow<str> {
    Regex::new(r"does-not-matter").unwrap().replace_all(s, "whatever")
}
```

But this doesn't pass `borrowck`. Instead, you could write `moo` like
this:

```rust
fn moo<F: FnOnce(&str) -> Cow<str>>(c: &mut Cow<str>, f: F) {
    let result = f(&c);
    match result {
        Cow::Owned(s) => *c = Cow::Owned(s),
        Cow::Borrowed(s) => {
            if !std::ptr::eq(s, &**c) {
                *c = Cow::Owned(s.to_owned())
            }
        }
    }
}
```

But the `std::ptr:eq` call here is a bit strange. Instead, after this PR
and the new guarantee, one can write it like this:

```rust
fn moo<F: FnOnce(&str) -> Cow<str>>(c: &mut Cow<str>, f: F) {
    if let Cow::Owned(s) = f(&c) {
        *c = Cow::Owned(s);
    }
}
```