Removes the fptr versions of filter & linear search.

Also tidies the API docs.
Remove the fptr versions of map.

Also tidy the API docs.
Removes the file headers for RXMap.
Removes the file headers for RXFold.
Remove the fptr variant of folding.

And, of course, tidy the docs.
Enumerable, not traversable.
Removes the fptr variants of RXMin/RXMax.
Tidying & fptr removal of RXConvolve.
Documentation comment cleanup.
Adds enumerators to the framework.

These are a (slight) generalization of traversals, designed to be
implementable with NSEnumerator, and based loosely on the description
of the hierarchy of Range abstractions given by Andrei Alexandrescu[1].

The selection of “enumerator” as a name is intended to evoke
NSEnumerator.

1: http://www.informit.com/articles/printerfriendly.aspx?p=1407357
Adds an enumerator over NSFastEnumeration.
Consume, rather than advance, the current object.
Count, not length, for finite enumerations.
Add an object buffer.
Rephrase exhaustion as emptiness.
Extracts RXMagnitude into its own header.
Intervals are structs.

There is an enumerator for them.
Adds a batching enumerator.
Interval enumerators are batched.
The enumerable protocol is once again traversable.