Function camel_to_snake 

pub fn camel_to_snake(s: &str) -> String

Convert camelCase or PascalCase to snake_case

Examples

assert_eq!(camel_to_snake("userId"), "user_id");
assert_eq!(camel_to_snake("deviceId"), "device_id");
assert_eq!(camel_to_snake("APIKey"), "api_key");
assert_eq!(camel_to_snake("HTTPSEnabled"), "https_enabled");
assert_eq!(camel_to_snake("user_id"), "user_id");