Implement the account storage API

[greenhat]

As discussed in #333

#[account_storage]
struct MyAccountStorage {

    #[storage_slot(0)]  
    foo: Word,

    #[storage_slot(1)]  
    pub_key: StorageVec,

    #[storage_slot(2)]  
    misc: Word,
}

The attribute macros generate getters and setters (MyAccountStorage::foo().get(), .set(value: Word)) for the fields which call get_item and set_item on the account storage.