Revamp to allow fine-tuning to control when and by how

Revamp to allow fine-tuning to control when and by how
much the table grows and shrinks.
(next_prime): Don't assert.
(hash_reset_tuning): New function.
(check_tuning): New function.
(hash_initialize): Accept and use new tuning parameter.
(hash_rehash): Rewrite, updating for tuning.
(hash_insert): Honor tuning semantics.
(hash_delete): Likewise.
From François Pinard.