Release-blocking TODO HS in tor-keymgr

MRs to be merged:

• !1321 (merged)
• !1312 (merged)
• !1262 (merged)
• !1337 (merged)

README.md:

• TODO hs: write more comprehensive documentation when the API ... (!1360 (merged))

key_specifier.rs:

• TODO hs: restrict the character set and syntax for values of this type (!1262 (merged))
• #[allow(clippy::unnecessary_wraps)] // TODO hs: remove (!1262 (merged))

key_type.rs:

• // TODO hs: this is subject to change
• // TODO hs: this is subject to change (!1334 (merged))

keystore.rs:

• TODO hs: the key_type argument here might seem redundant (!1334 (merged))
• TODO hs: try to fold this trait into EncodableKey. (!1334 (merged))

mgr.rs:

• TODO hs: we immediately return if one of the keystores is inaccessible. (!1334 (merged))
• TODO hs: would it be useful for this API to return a Result<Option<K>> here (i.e. the old key)? (!1334 (merged))

ssh.rs:

• // TODO hs: remove (!1321 (merged))
• todo!() // TODO hs (!1321 (merged))
• // TODO hs: implement (!1321 (merged))