Yet further feedback for "Non-exhaustive enums"
Yet further feedback for "Non-exhaustive enums"

- Add a "Comparison with other languages" section.

- Clarify up front that this only affects cross-module public enums.

- Mention the possibility of a compatibility checker.

- Link to Rex Fenley's counter-argument in favor of
  exhaustive-by-default.

- Include a code example for why `#invalid` isn't sufficient for
  testing default cases.

- Include "sealed"/"nonsealed" as a possible spelling (cf. Scala).

- Acknowledge "Drop `nonexhaustive`" as a considered alternative.

- Remove references to a "C++ proposal" for a "non-exhaustive"
  annotation. This one paper from last year hasn't gone anywhere yet.

- Link to the implementation PR.

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