2.26.146: restore W-C-U around perform-plan for backward compatibility.
Also, move entry-point into system and remove class bundle-system. I was tempted to have test-op call it, but then it's a problem when the base system has a non-test entry-point, so we probably need a separate test-entry-point slot. Update the default *asdf-debug-utility* to do more of the right thing. Update changelog. Tweak package to avoid triggering a bug in ABCL trunk.