more lifetime elision in generated code

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 52.45%. Comparing base (ab342b3) to head (34d57a3). Report is 117 commits behind head on master.

Files with missing lines	Patch %	Lines
capnpc/src/codegen.rs	0.00%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #532 +/- ## ========================================== + Coverage 51.64% 52.45% +0.81% ========================================== Files 69 70 +1 Lines 33735 34570 +835 ========================================== + Hits 17422 18135 +713 - Misses 16313 16435 +122 ```

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