Automatically check for license headers

[pepyakin]

rustfmt can help with this check. An example can be found in the https://github.com/paritytech/ink repo.

[Robbepop]

While rustfmt provides the feature to check for this we cannot easily copy what we do at the ink! project because ink! uses only the APACHE v2.0 license whereas wasmi uses either APACHE v2.0 or MIT license at the users choice.

I do not know how license headers in this scenario have to look like.