clean up iframe message passing code

per @hamilton's comment here: https://github.com/iodide-project/iodide/pull/1428#discussion_r251545494

One option to investigate: this is a pretty slim lib that seems to handle iframe communication using the style hamilton describes, as well handling and handshakes and stuff -- https://github.com/dollarshaveclub/postmate Maybe we could build off that?...

Start with the discussion linked from pull request #1428, then review the existing iframe message-passing code and the Postmate library mentioned in the issue. The issue does not name files, tests, or a specific entry point; completion would require agreeing on the cleanup scope and confirming that iframe communication and handshakes still work.

Written by the indexing model from the issue text.