Use std::unordered_multimap for BindingHash (#357)

This change can't really be done in isolation, since once we add members
with constructors to a struct, it is no longer has a trivial
constructor. This propagates through all types that use it, etc.

There are a number of changes that are ugly, but hopefully reduced the
amount of code that needed to change. In particular, I changed some
union members to pointers so they would stay trivially constructible.

Another tricky change is the handling of duplicate bindings: previously
we just relied on the fact that our hash implementation would be
consistent. A nicer solution is to display the duplicated bindings in
source order. There's probably a nicer way to do this; bikeshedding
welcome. :-)

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