Extract the iterable protocol to its own header.

Conflicts:
	Reducers.xcodeproj/project.pbxproj
	Reducers/REDConvolve.m
Add a typedef for iterators.
Implement iteration over fast enumerations.

Conflicts:
	Reducers/REDReducible.m
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.

Conflicts:
	Reducers/REDConvolve.m
Attributed strings are iterable.
Enumerators are iterable.
Eschew stray newline at EOF.
Merge pull request #35 from robrix/iterables

[WIP] Iterables