Loosen lifetime bounds on accessing named captures via `Index`.
Loosen lifetime bounds on accessing named captures via `Index`.

The current definition equates the lifetime of the strings being
matched/captured and the string used as the capture name.  The latter
is usually `'static` (string literals), which leads to confusing
[borrow-check errors][err] on reasonable-looking code.

This change declares the two lifetimes as separate; this should be a
strict increase in the set of `impl` instances, so it's a minor version
bump.

[err][]: https://gist.github.com/jld/476a13a00ad05bd99a63

rust-lang/regex,SeanRBurton/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,SeanRBurton/regex,SeanRBurton/regex,rust-lang-nursery/regex,SeanRBurton/regex,SeanRBurton/regex,rust-lang/regex,rust-lang-nursery/regex,golddranks/regex,rust-lang-nursery/regex,golddranks/regex,golddranks/regex,rust-lang/regex