Revert "WIP try combining PlatformHeaderInfo & HeaderInfo"

This reverts commit 100044295f.

crates/compiler/parse/src/header.rs

@@ -26,6 +26,8 @@ pub enum HeaderType<'a> {
     /// Only created during canonicalization, never actually parsed from source
     Builtin { generates_with: &'a [Symbol] },
     Platform {
+        /// usually `pf` - this is not actually something we parse out of the file.
+        shorthand: &'a str,
         /// the type scheme of the main function (required by the platform)
         /// (currently unused)
         #[allow(dead_code)]