1.0.13.11: ensure that sb-cover records don't clash with constants in user code

1.0.13.11: ensure that sb-cover records don't clash with constants in user code

        * If a code coverage instrumentation record is coalesced with another
          constant in the same file, the "we know what we're doing" comment
          in INSTRUMENT-COVERAGE about modifying constants no longer applies.
          Changes to the record will be reflected in the other constant too,
          which is probably not what the user expected.
        * Ensure that coalescing cannot happen by using a symbol internal to
          SB-C as the CDR of the record (instead of NIL).
        * Reported separately by Christophe Rhodes and Rahul Jain.