cross-package fnArgDocs. WARNING: changes .haddock binary format

While breaking the format, I took the opportunity to unrename the
DocMap that's saved to disk, because there's really no reason that
we want to know what *another* package's favorite place to link a
Name to was.  (Is that true? Or might we want to know, someday?)

Also, I added instance Binary Map in InterfaceFile.
It makes the code a little simpler without changing anything of
substance.  Also it lets us add another Map hidden inside another
Map (fnArgsDocs in instDocMap) without having really-convoluted
serialization code.  Instances are neat!
I don't understand why this change to InterfaceFile seemed to
subtly break binary compatibility all by itself, but no matter,
I'll just roll it into the greater format-changing patch. Done!

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