make criterion an optional rather than dev dep

criterion was making it take too long to compile tests, and there is no
way to turn it off

Cargo.toml

@@ -54,6 +54,9 @@ rand = { version = "0.7.3", optional = true }
 codemap = "0.1.3"
 peekmore = "0.4.0"
 wasm-bindgen = { version = "0.2.60", optional = true }
+# criterion is not a dev-dependency because it makes tests take too
+# long to compile, and you cannot make dev-dependencies optional
+criterion = { version = "0.3.2", optional = true }
 
 [features]
 default = ["commandline", "random"]
@@ -67,11 +70,12 @@ random = ["rand"]
 wasm = ["wasm-bindgen"]
 # Option: enable features that assist in profiling (e.g. inline(never))
 profiling = []
+# Option: enable criterion for benchmarking
+bench = ["criterion"]
 
 [dev-dependencies]
 tempfile = "3"
 paste = "0.1"
-criterion = "0.3.2"
 
 [profile.release]
 debug = true