* 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
