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/env/tock/mod.rs

@@ -1,4 +1,5 @@
 pub use self::storage::{TockStorage, TockUpgradeStorage};
+use crate::api::customization::{CustomizationImpl, DEFAULT_CUSTOMIZATION};
 use crate::api::firmware_protection::FirmwareProtection;
 use crate::ctap::hid::{CtapHid, CtapHidCommand, KeepaliveStatus, ProcessedPacket};
 use crate::ctap::status_code::Ctap2StatusCode;
@@ -80,6 +81,7 @@ impl Env for TockEnv {
     type UpgradeStorage = TockUpgradeStorage;
     type FirmwareProtection = Self;
     type Write = Console;
+    type Customization = CustomizationImpl;
 
     fn rng(&mut self) -> &mut Self::Rng {
         &mut self.rng
@@ -104,6 +106,10 @@ impl Env for TockEnv {
     fn write(&mut self) -> Self::Write {
         Console::new()
     }
+
+    fn customization(&self) -> &Self::Customization {
+        &DEFAULT_CUSTOMIZATION
+    }
 }
 
 // Returns whether the keepalive was sent, or false if cancelled.