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>

Cargo.toml

@@ -45,6 +45,11 @@ sk-cbor = { path = "libraries/cbor" }
 uuid = { version = "0.8", features = ["v4"] }
 openssl = "0.10.36"
 
+# We explicitly lock the version of those transitive dependencies because their
+# Cargo.toml don't parse with the nightly compiler used by Tock. Remove this
+# whole block once CTAP is a library.
+once_cell = "=1.14" # transitive dependency of openssl
+
 [profile.dev]
 panic = "abort"
 lto = true # Link Time Optimization usually reduces size of binaries and static libraries