Add Heyting algebras; better Boolean/Heyting algebra tests.

This commit formalizes the laws for Boolean and Heyting algebras, and
generalizes the BooleanAlgebra operators to Heyting algebras. It does not
change any of the import names--this could be a future change.

It also cleans up many 2.11 test warnings due to unused imports.
Can delete these lines, as we are defining defaults above.