Sentaku

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* upgrade attrs to at least 17.0.0 and s/convert/converter/

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* contextual property strictness
* freeze ImplementationName

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* fix the typo i introduced while overconfidently commiting the bits for v0.6

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* fix latent bug in legacy external implementation registration
by taking away multi value registration,
use multiple registration directives instead
* bring back a version of register_external_implementations_in
in order to keep legacy code working
* add a feature to allow non-strict method calls that allow nesting
* split Element behaviour into ElementMixing to allow reuse with different inheritance trees

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* switch registration to dectate
* require custom context classes for registration of implementations and overriding

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* add ImplementationContext.from_instances
* add contextual properties