Implement [more] app header formatting

Also, refactor out a utility method to format a Collection. This method can currently
replace some of the formatting done in module headers - but the goal is eventually
to be able to replace the code in fmt_list as well, such that there is 'one true way'
to format collections.

compiler/parse/src/header.rs

@@ -72,7 +72,7 @@ pub struct InterfaceHeader<'a> {
     pub after_imports: &'a [CommentOrNewline<'a>],
 }
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Copy, Clone, Debug, PartialEq)]
 pub enum To<'a> {
     ExistingPackage(&'a str),
     NewPackage(PackageOrPath<'a>),