Delay dictionary realization when it's enumerated

[dahlia]

Continued from #34. This make latency short when a Dictionary having unrealized internal pairs is enumerated. The solution is to delay its realization and just enumerate its internal key–value pairs. One trick here I used to avoid key duplication (note that unrealized keys don't guarantee their uniqueness) is simply comparing with the key from the previous iteration. This trick works because the enumeration is done after keys are sorted first.

Before:

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After:

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[moreal]

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