Add missing std subfeature dependencies

Author: roblabla

Cargo.toml

@@ -52,7 +52,7 @@ name = "key"
 
 [features]
 default = ["std"]
-std = []
+std = ["num-bigint/std", "num-traits/std", "num-integer/std", "num-iter/std", "rand/std", "byteorder/std", "failure/std", "subtle/std"]
 nightly = ["subtle/nightly", "num-bigint/nightly"]
 serde1 = ["num-bigint/serde", "serde"]
 expose-internals = []