v0.2.6

Exercise the assertion surface, the matchers and the runner's corners

An assertion that cannot fail turns a consumer's green suite into a claim
nobody checked, and most of the assert surface had never been called. Every
method is now asked a question it must accept and one it must refuse, the
matchers are checked for a non-empty failure message, and equals() is
measured on the types a plain key-walk gets wrong.

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Changes since v0.2.5.