Make upgrade smarter:
* only frob symbols when upgrading from before *oldest-forward-compatible-asdf-version*
* allow individual when-upgrade to override which version they apply to.
* tell SBCL to unintern bundle-op on old versions, because the redefinition
 causes an interim circularity with a not-yet-redefined subclass,
 for which SBCL preemptively raises an error.
Bump version to 3.1.0.84