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

Closes #14602.  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("")`.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/libregex/lib.rs b/src/libregex/lib.rs
index 05f853a851e..3fadba9583e 100644
--- a/src/libregex/lib.rs
+++ b/src/libregex/lib.rs
@@ -103,7 +103,9 @@
 //! let re = regex!(r"(\d{4})-(\d{2})-(\d{2})");
 //! let text = "2012-03-14, 2013-01-01 and 2014-07-05";
 //! for cap in re.captures_iter(text) {
-//!     println!("Month: {} Day: {} Year: {}", cap.at(2), cap.at(3), cap.at(1));
+//!     println!("Month: {} Day: {} Year: {}",
+//!              cap.at(2).unwrap_or(""), cap.at(3).unwrap_or(""),
+//!              cap.at(1).unwrap_or(""));
 //! }
 //! // Output:
 //! // Month: 03 Day: 14 Year: 2012
@@ -285,7 +287,7 @@
 //! # fn main() {
 //! let re = regex!(r"(?i)a+(?-i)b+");
 //! let cap = re.captures("AaAaAbbBBBb").unwrap();
-//! assert_eq!(cap.at(0), "AaAaAbb");
+//! assert_eq!(cap.at(0), Some("AaAaAbb"));
 //! # }
 //! ```
 //!