Add a rand-std feature

Author: elichai

.travis.yml

@@ -30,6 +30,7 @@ script:
   - cargo build --verbose --features=fuzztarget
   - cargo build --verbose --features=rand
   - cargo test --verbose --features=rand
+  - cargo test --verbose --features="rand rand-std"
   - cargo test --verbose --features="rand serde"
   - cargo test --verbose --features="rand serde recovery endomorphism"
   - cargo build --verbose

Cargo.toml

@@ -28,7 +28,8 @@ path = "src/lib.rs"
 [features]
 unstable = []
 default = ["std"]
-std = ["rand/std"]
+std = []
+rand-std = ["rand/std"]
 recovery = []
 endomorphism = []
 lowmemory = []