Changelog-None: `fuzz-hsm_encryption.c` hard codes the lengths
sizeof(struct secret) as 32 and crypto_pwhash_argon2id_PASSWD_MAX
as 4294967295. Replace the latter with the former to improve
readability and maintainability.
While at it, replace the `tal_free()` call on our secret key with
`discard_key()`. This has the benefit of testing `discard_key()`.
[ Changed from fuzz-hsm_encryption to fuzz-hsm_secret --RR ]
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