Add a rand-std feature

Author: elichai

Cargo.toml

@@ -29,7 +29,8 @@ path = "src/lib.rs"
 unstable = []
 default = ["std"]
 fuzztarget = []
-std = ["rand/std"]
+std = []
+rand-std = ["rand/std"]
 recovery = []
 endomorphism = []
 lowmemory = []