Macro exports

macro_rules! exports {
    (
    packages = $($x:ident),*;
    runtimes = {
      $($key:literal = $val:ident),*
    }
  ) => { ... };
}

Exports a leadlang package

use interpreter::exports;

exports! {
  packages = MyPkg1,MyPkg2;
  runtimes = {
    "rt1" = MyRuntime1
  }
}