Introduce a trait to abstract the CTAP environment

The end goal is to provide users with:
- the Env trait that they should implement
- the Ctap struct that they can use

src/env/mod.rs

@@ -0,0 +1,18 @@
+use crate::ctap::hid::ChannelID;
+use crate::ctap::status_code::Ctap2StatusCode;
+use crypto::rng256::Rng256;
+
+#[cfg(feature = "std")]
+pub mod test;
+
+pub trait UserPresence {
+    fn check(&self, cid: ChannelID) -> Result<(), Ctap2StatusCode>;
+}
+
+pub trait Env {
+    type Rng: Rng256;
+    type UserPresence: UserPresence;
+
+    fn rng(&mut self) -> &mut Self::Rng;
+    fn user_presence(&mut self) -> &mut Self::UserPresence;
+}