Function createScope

Creates a new Scope.

Ids start at 0 and increment monotonically, so all ids the scope hands out are pairwise distinct.

Creates a new Scope with mapped values.

Ids start at 0 and increment monotonically, so all ids the scope hands out are pairwise distinct; each is passed through mapper to produce the value returned. mapper runs once per id and its result is cached, so a repeat keyed lookup returns the same value (the same reference when mapper produces objects) without re-invoking it.