1.0.6.2: remove multiple layout-clos-hash slots

1.0.6.2: remove multiple layout-clos-hash slots

 * It seems that despite the claims of the paper "Efficient Method
   Dispatch in PCL" the multiple hash seeds yield a neglible benefit.

 * The soon-to-come thread safe cache also uses only a single hash
   value, so removing these now allows better performance comparisons:
   multiple hash values vs. single hash value vs. new cache.

 Actual work done mostly by Christophe Rhodes.