Implement Default trait

[LimpidCrypto]

Implement the Default for any struct it makes sense. The following example shows how much it makes it easier to define a struct:

let request = AccountInfo {
    account: "r3rhWeE31Jt5sWmi4QiGLMZnY3ENgqw96W",
    ..Default::default()
};

Instead of having to define all fields for AccountInfo the user can just define the account and leave the rest to ..Default::default().