1.0.46.33: constraint: Don't substitute REFs when replacement LEAF is not visible.

1.0.46.33: constraint: Don't substitute REFs when replacement LEAF is not visible.

  * Bug reported by Frank Duncan, additional diagnosis by Heka Treep,
patch review and revision by Nikodemus Siivola.

  * Introduce LEAF-VISIBLE-FROM-NODE-P, which verifies that the LEAF
about to be substituted is visible from the NODE to be modified.

  * Add LEAF-VISIBLE-FROM-NODE-P as an additional precondition to
calling CHANGE-REF-LEAF, which does the actual substitution.

  * And, for good measure, added a test to the test suite.