Follow-up: SpreadLayout (#995)

* implement const_fn Key::new constructor

* add ContractRootKey trait to ink_lang crate

* use qualified path

* re-export ContractRootKey trait

* automatically implement ContractRootKey for ink! contracts

* implement #[derive(SpreadAllocate)] macro & tests

* re-export #[derive(SpreadAllocate)] macro from ink_storage::traits

* automatically implement SpreadAllocate for the ink! storage struct

* extend execute_constructor to work with fallible ink! constructors

Note that this commit does not yet allow to write fallible ink! constructors.

* add initialize_contract utility method

* re-export initialize_contract utility method

* adjust ink! codegen for recent changes with execute_constructor

* fix clippy warning

* fix hunspell dictionary

* add missing docs to codegen types

* fix UI test

* remove automatic SpreadAllocate implementation

This is due to the fact that trivial_bounds feature is not yet stable.
Otherwise this automatically generated implementation would be totally fine.
We can re-add this feature once Rust adds trivial trait bounds.

* Satisfy OCD in test

* Remove mention of `enum`s from `derive_struct()`

* Remove outdated comment about Clippy lint

* RustFmt

Co-authored-by: Hernando Castano <hernando@hcastano.com>