Prerequisites for `validate_block` in Cumulus (#1926)

* Adds benchmark for direct/indirect wasm function calls

* Store the benchmark function pointer in a `Cell`

* Add some documentation

* Make function implementations exchangeable

* Add parachain stub

* Add macro for registering the `validate_block` function

* Make all functions replace-able by unimplemented

* Some more refactoring

* Adds tests for executing empty parachain block

* Work on a new test with empty witness data

* Don't exchange `ext_print_*` stuff

* Some cleanup and one more function for `validate_block`

* More tests and more functions

* Fixes after merging master

* Use `parity-codec` `derive` feature

* CHange implementation of `wasm-nice-panic-message`

* Move `parachain` stuff to cumulus

* Updated wasm files

* Integrate feedback

* Switch to `ExchangeableFunction` struct

* More fixes

* Switch to Cell and panic on multiple replaces

* Increase `impl_version`

* Fix shifting

* Make the API more verbose of `ExchangeableFunction`

* Increase `impl_version`