First step for generating host externals out of the function definition in `sr-io` (#3567)

* Adds new wrapper traits for wasm executor

* Add new crate `substrate-wasm-interface`

Thew new crate holds types and traits for the communicating between the
wasm runtime and the host.

* Rewrite externals with new macro etc

* Fix vec initialization

* Make executor tests working

* Remove unused code + warnings

* Introduce `Pointer` and `WordSize` for working with wasm

* Fix tests and compilation

* Fix compilation

* Apply suggestions from code review

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Review feedback

* Remove unused conversions

* Make each host function its own struct

`HostFunctions` now just returns these function structs. Each function
can be executed by using one of the function structs. The inherent host
functions are now moved to the "normal" host functions.

* Remove byteorder

* Add floating point types

* Make pointer interface more safe

* Add type alias for wasm-interface Result

* More review comments