Parse slot_id in use from token state (#546)

* Parse slot_id in use from token state

Support switching to multi-PIN feature and making the default slots 8.
But the command to switch to multi-PIN isn't exposed yet because the
implementation isn't ready.

Main change of this commit is to cache the slot_id in use inside token
state, and retrieve it from token state when needed.

* Fix once_cell dependency (#548)

* fixed version of once_cell

* fixes comments

* removes unnecessary fuzz dependency

* Fix styles

Co-authored-by: kaczmarczyck <43844792+kaczmarczyck@users.noreply.github.com>

libraries/crypto/Cargo.toml

@@ -25,6 +25,7 @@ regex = { version = "1", optional = true }
 # Cargo.toml don't parse with the nightly compiler used by Tock. Remove this
 # whole block once CTAP is a library.
 bumpalo = "=3.8.0" # transitive dependency of ring
+once_cell = { version = "=1.14", optional = true } # transitive dependency of ring
 
 [features]
 std = ["hex", "ring", "rng256/std", "untrusted", "serde", "serde_json", "regex"]