Modify `regex::Captures::{at,name}` to return `Option`

Hello! This is my first Rust patch, and I fear that I've probably skipped at least 7 critical steps. I'd appreciate your feedback and advice about how to contribute to Rust.

This patch is based on a discussion with @BurntSushi in #14602 a while back. I'm happy to revise it as needed to fit into the modern world. :-)

As discussed in that issue, the existing `at` and `name` functions represent two different results with the empty string:

1. Matched the empty string.
2. Did not match anything.

Consider the following example.  This regex has two named matched groups, `key` and `value`. `value` is optional:

```rust
// Matches "foo", "foo;v=bar" and "foo;v=".
regex!(r"(?P<key>[a-z]+)(;v=(?P<value>[a-z]*))?");
```

We can access `value` using `caps.name("value")`, but there's no way for us to distinguish between the `"foo"` and `"foo;v="` cases.

Early this year, @BurntSushi recommended modifying the existing `at` and `name` functions to return `Option`, instead of adding new functions to the API.

This is a [breaking-change], but the fix is easy:

- `refs.at(1)` becomes `refs.at(1).unwrap_or("")`.
- `refs.name(name)` becomes `refs.name(name).unwrap_or("")`.