Refactored the work with storage

[xgreenx]

One Storage trait for all contracts

Instead of defining the storage trait for each data structure, OpenBrush provides the openbrush::traits::Storage trait for all contracts. Removed derive macro at all. Simplified implementation of Storage trait.

It simplified the usage of the library:

/// Define the storage for PSP22 data, Metadata data and Ownable data
#[ink(storage)]
#[derive(Default, SpreadAllocate, Storage)]
pub struct SharesContract {
    #[storage_field]
    psp22: psp22::Data,
    #[storage_field]
    ownable: ownable::Data,
    #[storage_field]
    metadata: metadata::Data,
}

Instead of the old version with many storage traits:

/// Define the storage for PSP22 data, Metadata data and Ownable data
#[ink(storage)]
#[derive(Default, SpreadAllocate, PSP22Storage, OwnableStorage, PSP22MetadataStorage)]
pub struct SharesContract {
    #[PSP22StorageField]
    psp22: PSP22Data,
    #[OwnableStorageField]
    ownable: OwnableData,
    #[PSP22MetadataStorageField]
    metadata: PSP22MetadataData,
}

Helpers StorageAsRef and StorageAsMut traits

Added helpers StorageAsRef and StorageAsMut traits. If you implement the Storage trait those traits are implemented automatically for you. It simplified selection between several data storages in generic implementation. New versions:

fn foo<T: Storage<psp22::Data> + Storage<psp34::Data>>(instance: &mut T) {
    let psp22 = instance.data::<psp22::Data>();
    let psp34 = instance.data::<psp34::Data>();
}

Old version:

fn foo<T: PSP22Storage + PSP34Storage>(instance: &mut T) {
    let psp22 = PSP22Storage::get(instance);
    let psp34 = PSP34Storage::get_mut(instance);
}

OccupiedStorage and OccupyStorage traits

Added the OccupiedStorage trait to allow generics in the data structures. Also, it is an additional guard not to occupy the same storage key. Each upgradeable data in OpenBrush implements OccupyStorage with provided storage key into openbrush::upgradeable_storage.

OccupiedStorage is implemented by derive Storage macro. So if you want to use it then the data type should implement OccupyStorage.

The simplest way to not worry about it is to use openbrush::upgradeable_storage during the definition of your structure and use Storage derive in the main contract.

Minor improvements

Removed module duplicating in the naming of data, traits, etc.
Each main data structure is now named Data instead of PSP22Data or PSP34Data.
Each internal trait is now named Internal instead of PSP22Internal or PSP34Internal.

If you want to have several Data in scope or several Internal traits, you can use namespace to specify it:

struct Contract {
    foo: psp22::Data,
    bar: psp34::Data,
}

impl psp22::Internal for Contract {}
impl psp34::Internal for Contract {}

---

Updated storage key to be u32(soon, a new version of ink! will use it as the main key instead of [u8; 32]). Provided a new macro openbrush::storage_unique_key that generates a storage key based on the module path to the data structure.

---

Improved exporting of all stuff from contracts. Now when you import openbrush::contracts::psp22::*, it also exports psp22 itself. So you can use psp22::Data or Data. Usage of psp22::Data is better because it is more readable.

---

Updated documentation. Removed common parts of each example documentation into the Overview section. Renamed storage of examples into Contract.