Typedef the enumerator state.
Parameterize refilling with the state.
Parameterize refilling with the collection.
Refilling can now be static.
Add Obstruct.
Remove the array convolver.
Implements convolution in terms of iterables.
Extract the iterable protocol to its own header.
Add a typedef for iterators.
Move the fast enumeration iteration into its own file.
Publish the fast enumeration iterator.
Add a function to enumerate an iterator.
Arrays are iterable.
Sets are iterable.
Ordered sets are iterable.
Dictionaries are iterable.
Strings are iterable.

This also corrects the previous implementation of string iteration for
emoji and other code points > 65535.
Attributed strings are iterable.
Enumerators are iterable.
Convolution takes iterables.