[R6RS] optional hash-table-clear! k argument

[dyb at cs.indiana.edu]

I have made a few wording changes and added a rationale for immutable
hash tables in hashtable.tex.  I also took the liberty of making a change
to the interface that might be controversial (though I doubt it), which is
to extend hash-table-clear! to take an optional k argument by analogy
with the optional k argument to make-eq-hash-table, make-eqv-hash-table,
make-hash-table, etc., i.e.:

  If a second argument is given, the current capacity of the hash table is
  reset to approximately \var{k} elements.

If anyone wants to object, please do so and I'll put it up for a vote.

Kent