Implement `Replacer` also for closures that return a type which depends
on lifetime of the argument. This allows performing a no-op replacement
where the closure returns the whole match, for example, without needing
to clone the captured string.

Fixed code formatting using rustfmt.

Less generic implementation, which results in a better documented API.

Do not use the same lifetime for reference and type parameter of
`Captures`.

Hide `ReplacerClosure` trait as an implementation detail and describe
for which closures the `Replacer` trait is implemented in the
documentation instead.

Added documentation tests.

Fixed error in documentation comment in private module.

Ignore documentation test that needs unstable Rust.

Do not use closure lifetime binder but helper function for coercion in
documentation test.

Added documentation comment on `Replacer` impl for closures.

Removed unnecessary import of `Replacer` in doc test.

Use same lifetime for reference and type parameter of `Captures` (again)
because `Captures<'a>` is covariant over `'a`. This covers closures that
accept a `&'a Captures<'b>` as argument and have a result type that
depends either on `'b` or `'a`. Documentation was updated and
corresponding test cases have been added to `tests/misc.rs`.

A link to the blanket implementation has been added to the
"Implementation by closures" section of the documentation on `Replacer`.

Refactored code to be able to use two different lifetimes `'a` and `'b`
for the `&'a Captures<'b>` argument while allowing the return type to
depend on either `'a` or `'b`.

Do not mention unstable features in documentation. Do not include
coercing helper function in example code.