chore: turn Mapping into a wrapper type

[0xrusowsky]

tackles the 1st:

• Storage-related TODO's #32

since Mapping is now a wrapper type, i had to split StorageStorable into 2 types of traits: based on whether there is direct interaction with the db or nor.

then, i created 2 traits (DirectStorage, KeyValueStorage) to signal the public fn read(), fn write() API that devs will use. Maybe skill issue, but i didn't mange to do it with a single trait. Open to feedback or a better design if this one feels too convoluted.

I tried to summarize my logic in the docs:

///  STORAGE TYPES:
///  > Must implement the following traits:
///     - `StorageLayout`: Allows the `storage` macro to allocate a storage slot.
///  > Must implement one of the following traits, for external consumption:
///     - `DirectStorage`:  Exposes read and write capabilities of values that are directly accessed.
///     - `KeyValueStorage`:  Exposes read and write capabilities of values that are accesed by key.
///  > Unless it is a wrapper type (like `Mapping`) it must implement the following traits:
///     - `StorageStorable`: Allows db storage reads and writes with abi de/encoding.