Base Key on [u8; 32] (#971)

* implement new Key2 primitive that is based on [u8; 32]

* implement Key::add_assign_u64_using method

* integrate new Key type into the rest of the ink! codebase

* rename key2.rs -> key.rs

* fix some Wasm build bugs with new Key type integration

* add #[inline] to Key::add_assign_u64_using method

* optimize Key add assign methods

Somehow those new implementations optimize better ...

* apply rustfmt

* fix hunspell issues

* fix clippy issues

* improve LazyArray and LazyIndexMap impls

* rename add_assign_u64_using to add_assign_using and make it generic

Generic over T where T: Into<u64> so it accepts u32 as well etc.

* remove unnecessary cast