Before:
test test::bench_nonpod_nonarena ... bench: 62 ns/iter (+/- 6)
test test::bench_pod_nonarena ... bench: 0 ns/iter (+/- 0)
After:
test test::bench_nonpod_nonarena ... bench: 158 ns/iter (+/- 11)
test test::bench_pod_nonarena ... bench: 48 ns/iter (+/- 2)
The other tests show no change, but are adjusted to use the generic
return value of `.iter` anyway so that this doesn't change in future.
In line with the dissolution of libextra - #8784 - moves arena to its own library libarena.
Changes based on PR #11787. Updates .gitignore to ignore doc/arena.