contracts: Refactor `trait Ext::*_storage_transparent` functions (#13600)

* Refactor _transparent methods

rewrote commits, stashed the typo changes to remove some diff noise
fixed my unverified email commit

* remove type alias

* Get rid of From<Fix/VarSizedKey> impl blocks

* Get rid of KeyType impl block

* remove unnecessary Key export

* Update frame/contracts/src/exec.rs

Co-authored-by: Sasha Gryaznov <[email protected]>

* PR review comment

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Co-authored-by: Sasha Gryaznov <[email protected]>