I made some serialization updates for archives to fix an issue I accidentally introduced with my DCE update (sorry 😥) . The DCE data wasn't being serialized because of how I encapsulated it and kept the fields un-exported. This meant an archive that was reloaded from memory would have no DCE info and think every Decl in it are always alive. This was discovered as I was exploring what kind of information we'd need to add to an archive for cross package generic resolution.

• I fixed the serialization of DCE whilst keeping the encapsulation.
• I fixed the "TODO"s in archives around getting access to that actual types.Package and token.FileSet instead of having byte arrays.
• I added some tests to the compiler that checks the JS after reloading from memory results in the same JS as without reloading, including DCE.
• I changed how build time and source mod time is passed into cache. This is partially done because reloading the types.Package from the byte array is now done earlier and we needed to still skip that if the archive is out-of-date.

This is related to #1013 and #1270