Add requirement of std to from_str feature

Author: jhbruhn

Cargo.toml

@@ -16,7 +16,7 @@ readme = "README.md"
 
 [features]
 std = []
-from_str = ["regex"]
+from_str = ["regex", "std"]
 
 [dependencies]
 serde = { version = "1.0", optional = true, default-features = false, features = ["derive"] }