Consider moving criterion benchmarks back to libtest

[garious]

Devs are not seeing good ROI on Criterion, and would prefer to keep cargo watch -x bench running as fast as possible. We should consider one of:

• moving the benchmarks in the benches folder back to libtest
• add a flag that runs them with libtest by default and optionally runs them with Criterion for improved precision
• make libtest and criterion benches coexist, but the criterion ones only added to the build when a flag is present (i.e. --features=criterion).

[garious]

Fixed in #1032. After RustConf, it became clear that we don't need both benchmarking suites to coexist, that criterion is just too heavyweight.

And that benchcmp gives us the dev workflow that made criterion so appealing: https://github.com/BurntSushi/cargo-benchcmp