feat: Custom StorageAccess implementation (packing)

[enitrat]

In this example, we will implement a custom StorageAccess implementation for a struct.
We will demonstrate how we can pack multiple fields together instead of storing them in different storage addresses.

You will optimize storage by packing values together using bitshifts and bitmasks inside the read_at_offset_internal and write_at_offset_internal methods of the StorageAccess trait.

For example,

struct Vote{
   proposal_id: u8,
   timestamp: u32, 
   value: bool,
}

[julio4]

Updated in #62