Update Reflexivity law of Ord

[JamieBallingall]

Description of the change

Fixes #300 by updating the Reflexivity law of Ord to read:

-- | - Reflexivity: if `a == b` then `a <= b`

which better connects Ord to Eq and eliminates an unintuitive case as described in the issue.

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Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• [X] Linked any existing issues or proposals that this pull request should close
• [X] Updated or added relevant documentation
• [X] Added a test for the contribution (if applicable)