[nfc] kj::HttpHeaders parse simple benchmark

[mikea]

No description provided.

[kentonv]

Could we put KJ benchmarks in KJ? There is a benchmarking framework in KJ. (See http-over-capnp-perf-test.c++ for example.)

[mikea]

Could we put KJ benchmarks in KJ? There is a benchmarking framework in KJ. (See http-over-capnp-perf-test.c++ for example.)

I'll take a look. Meanwhile the reason for keeping them here might be build infrastructure: I plan to capture these in every TC build (see #1063 for example).