Add a lifetime to Replacer

[sgrif]

The current implementation requires any implementations of Replacer be
valid for all possible lifetimes. However, this is neither required nor
is it always desirable. For example, a user who wishes to use the
lifetime of the input text as some part of the return type when
interacting with these APIs would end up with a closure that is valid
for one specific lifetime.

A concrete example of where this was problematic was described in
#775

Any future code that does require a Replacer which is valid for all
lifetimes can do so with HRTB:

where
    for<'a> R: Replacer<'a>

or R: Replacer<'_> in some contexts

[sgrif]

It looks like this might be a breaking change, since Replacer appears to be part of the public API (at least it shows up on docs.rs). Unfortunately I don't think there's any way we can work around this. The scope of the breakage would be anybody who is either implementing or using this trait, and the fix would be similar to this PR, which is what the compiler will suggest (typically adding <'_>)

[BurntSushi]

Thanks so much! Yeah, the breaking changes means this can't land until regex 2.0.0. I filed #777 to track this. It links to this PR, so I can bring this patch in whenever that happens. But I'll close this for now. :-) Thanks again!

[sgrif]

No worries. Glad I could help <3