Absence info at run-time

[David Feuer]

Suppose `f` doesn't use its first argument. When forming the thunk (or
partial application) `f a`, we don't need to record `a`. What if instead of
arity, we store a bitmap used/absent arguments, terminated by a 1 bit?
Could we then get rid of "stupid thunks" like `(const a) b`?
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