Consider memoizing forked continuations

[mariusae]

Currently, when a continuation node is forked, it is not canonicalized. This is done mostly due to simplicity, since doing this properly would require us to rewrite pointers to the canonicalized node. This does imply redundant computation, but never happens for expensive computations like exec.

However, we should consider memoizing their results so that continuations are not needlessly recomputed.

See also: #28