Also simplified error to string conversion.

It's used because of static size nature of it.

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.

Uses a different file for the C structs of Libretro.
Makes it easier to manage the structure these structures.

Was creating major memory issues, including in the save of state files.

Adds warning on failing to set callbacks.

Should allow simple testing of the core.

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.