1.0.5.28: new contrib: sb-cover, a code coverage tool

1.0.5.28: new contrib: sb-cover, a code coverage tool

         Still somewhat experimental, so it might be changed incompatibly once
         people start actually using it. Or maybe even removed, if it's not
         found to be useful in practice. More documentation on interpreting
         the output and on the limitations of the coverage tool will be
         committed later.

         * Compiler
           * Instrument all forms found in the original source code
             when the store-coverage-data optimization quality is larger
             than 1.
           * The instrumentation is fairly lightweight; just literal conses that
             are modified when the form is executed.
           * This requires hacking the compiler to allow modifying literals
             when we know what we're doing.
           * Add some extra source location tracking capabilities to handle
             non-cons source forms better (though still not perfectly).
         * Contrib
           * Get the instrumentation data from the compiler, and create
             a aggregate report + pretty annotate source code reports
             (as html) from it.
           * The latter uses a modified version of the source code tracking
             read-table from Swank, written by Helmut Eller.