summon sealed trait children definitions for Element Encoder/Decoder instead of auto-deriving

[mpipalovic]

Fixes #46

Notice that this is fixed solely by removing the LazySummon; as the only actual place to get an instance of LazySummon was from the ElementDerivedInstances. Meaning, alternative way of making this whole thing work (or fixing the issue at hand) would be by just adding the following:

      given [T: ElementEncoder]: LazySummon[ElementEncoder, T] = new:
        def instance = summon[ElementEncoder[T]]

      given [T: ElementDecoder]: LazySummon[ElementDecoder, T] = new:
        def instance = summon[ElementDecoder[T]]

(you can try this with code from master, and adding the new test with and without the code above)

so e.g. whenever there is a ElementEncoder[T], there would also be an LazySummon[ElementDecoder, T].

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But since the name of the LazySummon is kind of misleading

• since it's not really doing any summoning, but instead it's more like LazyDefaultDerived
• plus, if we add this code above - then it's no longer even Lazy in any way

So, it just becomes a wrapper that's making things more confusing for no reason with no purpose, so I just removed it

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I left the current auto derivation logic for Enums. Also, notice that in case there exists an user provided instance for one of enum children - it would be used (since there's no longer this LazySummon indirection)

[valentiay]

Hi! Thanks for your contribution, this is an important fix