Fix bug 47d (DEFGENERIC must signal PROGRAM-ERROR when
attempting to create a generic function with the same name as a
... sounds easy, huh? No.
... make COMPILER-ERROR not inherit from ERROR any more, so that
user handlers don't (wrongly) claim to handle it;
... establish a handler for COMPILER-ERROR around the evaluator
that delegates to the compiler handlers if present, but
handles them itself if not...
... by signalling an error from a new internal restart, to allow
user handlers for ERROR and friends a chance to run.