Introduce Customization struct (#458)

* Introduce Customization trait

* Introduce Customization trait including the customization accessors
  that control various behaviors.

* Expose Customization through a getter API in Env, and make the code
  that directly access the constants currently switch to accessing the
  customizations via Env.

* TockEnv's customization getter implementation directly returns the
  reference of the global DEFAULT_CUSTOMIZATION constant, so the
  constant values are still inlined and dead code won't be compiled.

* We'll add the customizations from global constants to the struct
  one-by-one, only MAX_MSG_SIZE in this commit.

* Small fixes

* Fix deploy script
* put is_valid under std gate

src/lib.rs

@@ -88,6 +88,11 @@ impl<E: Env> Ctap<E> {
         &mut self.hid
     }
 
+    #[cfg(feature = "std")]
+    pub fn env(&mut self) -> &mut E {
+        &mut self.env
+    }
+
     pub fn process_hid_packet(
         &mut self,
         packet: &HidPacket,