macros for init and callable methods

[geofmureithi]

Is your feature request related to a problem? Please describe.
In almost all our contracts we have this code:

pub fn init(env: Env, some: Params) {
        let storage = env.storage().persistent();
        if storage.get::<_, ()>(&DataKey::Initialized).is_some() {
            panic_with_error!(&env, Error::AlreadyInitialized);
        }
}

pub fn callable_method(env: Env, some: Params) {
        let storage = env.storage().persistent();
        if storage.get::<_, ()>(&DataKey::Initialized).is_none() {
            panic_with_error!(&env, Error::NotInitialized);
        }

Describe the solution you'd like
We could have some utilities macros:

#[init]
pub fn init(env: Env, some: Params) { ... }
#[must_init]
pub fn callable_method(env: Env, some: Params) { ... }

Additional context
Of course the naming can be different but hopefully this gets the point home.

[eloylp]

This looks great ! My vote for moving this forward.