[CM] iterating over a hash table

[Bill Schottstaedt]

> In the s-7 documentation, it says that a hash-table is a vector of 
> alists and can be iterated over by vector-ref.

oops -- thanks for pointing this out.  Now I can't decide whether to
re-enable vector-ref of a hash-table or provide hash-table-for-each --
any strong opinions?