* 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
We used to only check that the length was not too big but didn't check that the
starting offset wasn't too big. We want the end offset to not overflow.
