Be more clear around ratio serialization

* Use 'ratio' instead of 'rational' in some names
 * Signal a more descriptive error from WRITE-RATIO-AS-FLOATING-POINT
 * Add CONTINUE restarts to WRITE-RATIO-AS-FLOATING-POINT in case
   the ratio cannot be serialized without a loss of precision, and
   *SILENTLY-TRUNCATE-RATIOS* is false.

as a separate patch:
* Turn off *silently-truncate-ratios* by default