Discrepancy `hs` for keys, `hss` for non-key data

After #1183 (closed), and when we use !1935 (merged), we'll have hss for "storage" and hs for keys - see below.

We should make these consistent. I think we should change hs to hss in KeySpecifier impls.

MUST because this will be an incompatible change.

/home/rustcargo/.local/share/arti/hss
/home/rustcargo/.local/share/arti/hss/ztest
/home/rustcargo/.local/share/arti/hss/ztest/iptpub.json
/home/rustcargo/.local/share/arti/hss/ztest/ipts.json
/home/rustcargo/.local/share/arti/hss/ztest/iptreplay
/home/rustcargo/.local/share/arti/hss/ztest/iptreplay/8515aa9a4e0d242d8a9b2115f7c4e7eb3a7abc0a34bc46197d4dbc993b0a8ce3.bin
/home/rustcargo/.local/share/arti/hss/ztest/iptreplay/0625d5fabb9b1a7665d6a8c9ee05c745ec7380c5f9dbba3de4cb7aee5d55d398.bin
/home/rustcargo/.local/share/arti/hss/ztest/iptreplay/965e288fd6720de8bffd4509571e52846e5130658fde99447bd54aae5268a478.bin
/home/rustcargo/.local/share/arti/hss/ztest.lock
/home/rustcargo/.local/share/arti/keystore
/home/rustcargo/.local/share/arti/keystore/hs
/home/rustcargo/.local/share/arti/keystore/hs/ztest
/home/rustcargo/.local/share/arti/keystore/hs/ztest/KS_hs_blind_id+19751_1440_43200.ed25519_expanded_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/KS_hs_desc_sign+19751_1440_43200.ed25519_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/KS_hs_blind_id+19752_1440_43200.ed25519_expanded_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/KP_hs_id.ed25519_public
/home/rustcargo/.local/share/arti/keystore/hs/ztest/KS_hs_id.ed25519_expanded_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/ipts
/home/rustcargo/.local/share/arti/keystore/hs/ztest/ipts/k_hss_ntor+8515aa9a4e0d242d8a9b2115f7c4e7eb3a7abc0a34bc46197d4dbc993b0a8ce3.x25519_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/ipts/k_sid+8515aa9a4e0d242d8a9b2115f7c4e7eb3a7abc0a34bc46197d4dbc993b0a8ce3.ed25519_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/ipts/k_hss_ntor+0625d5fabb9b1a7665d6a8c9ee05c745ec7380c5f9dbba3de4cb7aee5d55d398.x25519_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/ipts/k_hss_ntor+965e288fd6720de8bffd4509571e52846e5130658fde99447bd54aae5268a478.x25519_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/ipts/k_sid+965e288fd6720de8bffd4509571e52846e5130658fde99447bd54aae5268a478.ed25519_private
/home/rustcargo/.local/share/arti/keystore/hs/ztest/ipts/k_sid+0625d5fabb9b1a7665d6a8c9ee05c745ec7380c5f9dbba3de4cb7aee5d55d398.ed25519_private