Fix NLL warnings on 0.2.x versions

Uncovered by crater in https://github.com/rust-lang/rust/pull/63565, this crate does not build on the 0.2.x versions with NLL turned on fully (it's currently in migration mode).

For example, see https://crater-reports.s3.amazonaws.com/pr-63565/try%2375eff020d0923c035c2fe220db4a0465cd847048/reg/clipping-0.1.1/log.txt as a sample error log.

I can make a patch which adds the appropriate annotations if that'd be helpful, though I think it should be fairly straightforward to add the relevant lifetime annotations if I'm not missing something.