Raku: ♪ USAGE() in a module, yeah ♪

mainusage.memo

As explained in this talk by lizmat,
<https://www.youtube.com/watch?v=VoxWnNJ0gTI&t=8440s>, to speed up the
launch time of a Raku script, one can write all of its MAIN logic in a
module, make sure sub MAIN is exported, and create a script that
simply "use"s the module (the talk is more general and worth
watching). The launch time speedup will occur once the program has
been run at least once, since its modules will have been precompiled,
as opposed to the script itself.

But what if we want to have a custom USAGE() function? We can write
one in the script and it wll be invoked (if necessary), but we'd
prefer to benefit from having it too precompiled. The solution is to
also place it in the module and to export it, in the same way MAIN()
is. For example:

    /tmp/myscript.raku:

        #!/usr/bin/env raku

        use lib $?FILE.IO.dirname;
        use MyModule;

    /tmp/MyModule.rakumod:

        proto sub USAGE (|) is export {*}
        multi sub USAGE { note "Command line arguments: {@*ARGS}" }

        proto sub MAIN (|) is export {*}
        multi sub MAIN ($foo) { say "Got foo: $foo" }

That's it.