Fix lifetimes for subcapture iterators in str regexes

[birkenfeld]

So that strings extracted from a Captures::iter_named can be used
after the captures struct is dropped.

Note: I started with the re_unicode API, introducing new lifetimes. Then I discovered that it was already correct in re_bytes, but I think the new lifetime names are more consistent, so I changed to 'c there.

The 'c' lifetime in the iter_pos definitions is elidable, I can do it that way if you prefer.

[BurntSushi]

@birkenfeld Thanks! This is issue #168. When I wrote the bytes API, I fixed this issue. However, the addition of a type parameter to a public type is a breaking change, so I'm holding off on making this change until the next semver bump.

[birkenfeld]

Ah right. I was only thinking about the methods, where there is no breakage, but the type change itself is indeed incompatible.

Well, I guess you'll make a 0.2 (or 1.0?) bump eventually.

[BurntSushi]

@birkenfeld Yeah, #168 is on the 1.0 milestone. :-)