Add support for tracking dollop entry membership in dollop and retrieving a dollop entry's fallthrough in a dollop.

Broken version being committed, so will can commit his broken version so we can both finish our hacking without dealing with repo conflicts.

check for matching types on AddObserver

1. Refactor ZiprUntypedOption_t -> ZiprOption_t
2. Add observee field: this will let an observer
track its observee. When () or == (etc) are called,
they should pass control to their observee.
3. Add type checking for AddObserver.

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This reverts commit d781da1f2ca0b6deb1eb167704458324cf4c81b0.

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