rafactor: Add trait types

[febo]

Problem

There are currently two issues on how account types are loaded:

1. The from_bytes method does not validate whether the byte slice has the expected size.
2. There is no option to validate whether the account type is initialized or not. This leads to duplicating the validation code across processors.

Solution

Improve the logic to load accounts by using a set of traits:

• RawType: marker trait for "raw" types.
• Initializable: types that can be initialized.

Additionally, a set of load_* helpers facilitate loading account data using these traits.