Ensure

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- Import ABCs from collections.abc. Fixes 27.

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- Return type of the incorrect value in EnsureError (28)

- Move implmentation into submodule to clean up namespace

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Length assertion helpers

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Remove unneeded dependency

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- Pass format args directly to format in or_raise (may need further work to pass custom args to factory)
- Changes to test infrastructure: linter, etc.

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- Add ``.satisfies(predicate, *args)`` (16).