Contracts: make benchmark dependencies optional in `std` feature (#2576)

`wasm-instrument` and `rand` are optional and only used in benchmarking,
so should not be pulled in by default as part of the `std` feature.

substrate/frame/contracts/Cargo.toml

@@ -88,7 +88,7 @@ std = [
 	"pallet-proxy/std",
 	"pallet-timestamp/std",
 	"pallet-utility/std",
-	"rand/std",
+	"rand?/std",
 	"scale-info/std",
 	"serde",
 	"sp-api/std",
@@ -97,7 +97,7 @@ std = [
 	"sp-keystore/std",
 	"sp-runtime/std",
 	"sp-std/std",
-	"wasm-instrument/std",
+	"wasm-instrument?/std",
 	"wasmi/std",
 	"xcm-builder/std",
 	"xcm/std",