Boot criterion (#1032)

* Revert benchmarks back to libtest

Criterion has too many dependencies, it's execution as slower, and
we didn't see the kind of precision we had hoped for to use it to
block CI builds.

* Ignore benchmarks that take more than a few milliseconds per iteration

* Revert "Ignore benchmarks that take more than a few milliseconds per iteration"

This reverts commit b87cdf6ef4.

* Don't run benchmarks in CI

They are already built in the nightly build. Executing them in CI
doesn't add much value until the results are precise enough to act
on.

ci/test-bench.sh

@@ -2,7 +2,7 @@
 
 cd "$(dirname "$0")/.."
 
-ci/version-check.sh stable
+ci/version-check.sh nightly
 export RUST_BACKTRACE=1
 
 _() {
@@ -10,4 +10,4 @@ _() {
   "$@"
 }
 
-_ cargo bench --verbose
+_ cargo bench --features=unstable --verbose