Core: More convenient data-traits

[mawildoer]

Description

Add more convenient traits for simple applications.

This convenience comes from allowing decless (declaration-less) implementations, which actually have an implicit declaration.

Checklist

Please read and execute the following:

• My code follows the coding guidelines of this project
• My PR title is following the contribution guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I ran Black to format my code

Code of Conduct

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[iopapamanoglou]

Very clean!