hashing: overhaul part 2.

In this commit we feature-complete the seeded hashing
implementation. The *hash-seed* special variable is provided
from which newly created hashes take their seed.  The
make-hash and hash-equal functions get an optional seed
argument. A function called gen-hash-seed is provided for
obtaining a randomized seed.

* hash.c (hash_seed): New macro.
(hash_seed_s): New symbol variable.
(make_seeded_hash): New function, made from make_hash.
(make_hash): Reduced to wrapper around make_seeded_hash.
(hash-equal): Take seed argument.
(gen_hash_seed): New function.
(hash_init): Initialize hash_seed_s and register *hash-seed*
special variable. Re-register make-hash with new optional
parameter, with regard to make_seeded_hash.
Re-register hash-equal with new optional parameter.
Register gen-hash-seed.

* hash.h (make_seeded_hash): Declared.
(hash_equal): Declaration updated.

* txr.1: Documented optional seed arguments of make-hash
and hash-equal. Documented new *hash-seed* variable
and gen-hash-seed function.

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