Support for read and write.

Will allow storage of serialized state for each "device".

This change created multiple traits for state info loading.

Also simplified error to string conversion.

Made read MMU a non mutable operation.
Added support for CPU description.
Added Display support for CPU.
Extracted type definition for `Instruction`.
Support for diag_gb!() - uses global unsafe approach, which is not problematic for the purpose.
Inline fetch() operation in cpu clock.
Moved pedantic validation up in the CPU.
Fix simple diag issue.
Changed naming of read and write unsafe ops to raw.
New pedantic asserts.
Fix an issue with HDMA destination address prevent CHIP-8 ROM from
working.

Includes simple example in main gb.rs file.

CGB is now going to boot using Boytacean ROM by default.
Allows better image and open source only boot.

Clarifies the boot state handling.

Added more pedantic validations preventing simple dummy data from being loaded.

Support for some basic Python testing.
Handling of loading error with proper error propagation.

Made the testing of the timer work.

Makes it consistent with the WebAssembly naming conventions.

Makes use of the `Error` enum to handle errors. This is a better way to handle errors in Rust. It is also more idiomatic.