All: normalize license notice in all *.rs files

[tertsdiepraam]

Since we had some trouble finding all authors in #5197, I thought it'd be nice to normalize all notices to exactly the same string. This meant place the comment all at the very top of the file and removing * symbols or additional spaces and other inconsistent formatting. This change should make it easier for us to check that all files have the notice or to change it if we are ever inclined to do so. Also it's just more satisfying that they're all the same :)

You can check that all files are correct with this nu oneliner:

glob src/**/*.rs tests/**/*.rs | filter { |it| not ($it | str ends-with "main.rs") } | each { |it| open --raw $it | lines | range 0..3 | str join "\n" } | uniq

(The main.rs files are omitted but we could add the notice there too)

[sylvestre]

much better, thanks