rfi: hash set

[Andy Wingo]

Hello,

Currently the symbol table takes up twice as much memory as it needs to,
because it is a hash table instead of a set. (The difference being that
the buckets in a set don't need to be pairs.)

We don't actually have a good set data type implementation, and I'm sure
people have opinions about this, so if anyone has the time, an
implementation would be appreciated. Name it hashset.[ch] and make sure
it handles the weak reference case.

Thanks! :) (Hey, it's worth a try :)

Andy
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