Substrate runtime interface 2.0 (#4057)

* Adds first version of traits for generating the host functions

* First steps of the procedural macro

* Implements generation of the host extern functions

* Prefix ext host function with snake case trait name

* Implement host functions implementation on the host

* Change `HostFunctions` interface

* Implement `HostFunctions` for tuples

* Make `WasmExecutor` generic over the host functions

* Begin to add a test and make it compile

* Make the test succeed

* Add test to ensure that host functions are not found

* It's alive! Make the `set_storage` test work

* Add test for mutable references

* Code cleanup and documentation etc

* Add marker trait for types that should be passed as SCALE encoded

* Inherit the visibility from the trait and more improvements

* More impls and move them into their own file

* Code simplification by dropping one trait

* Give it a better name

* Implement traits for arrays

* Refactor cod...