Implement a simple lazy proxy.
Revert "Move lazy evaluation into the combinators."

This reverts commit d3877868c0cc9ce3c07a8f9440464c30f8f327fb.
Revert "Add a header with delay/force macros."

This reverts commit e00665087447a72e0c3caea3e228465deff741e3.
Set the forced value in -force.
HMRDelay is copyable.
HMRDelay() is typesafe.
-isEqual:, always -isEqual:, never ==. Never.
Fewer explicit delays.
No more lazy combinator type.
Be a little more cautious about forcing.
Specialize a Delay subclass for combinators.
Split up combinator nouns & verbs.

Nouns in the protocol, verbs in the class.

Fixes #40.
Push the delay combinator into its own class.
Break a reference cycle.
Merge pull request #44 from robrix/lazy-proxies

Lazy proxies