Organize DataStructures into categories (#218)

Sources/DataStructures/CollectionExtensions.swift → Sources/DataStructures/Extras/CollectionExtensions.swift

@@ -5,6 +5,7 @@
 //  Created by James Bean on 6/29/18.
 //
 
+// TODO: Consider moving to the `Algorithms` module.
 extension Array {
 
     /// - Returns: Array with the `element` appended.

Sources/DataStructures/Homogeneity.swift → Sources/DataStructures/Extras/Homogeneity.swift

@@ -7,6 +7,7 @@
 
 import Destructure
 
+// TODO: Consider moving to the `Algorithms` module.
 extension Sequence where Element: Equatable {
 
     /// - Returns: `true` if there are one or fewer elements in `self`, or if all elements in

Sources/DataStructures/RangeProtocol+Algebra.swift → Sources/DataStructures/Extras/RangeProtocol+Algebra.swift

@@ -7,6 +7,7 @@
 
 import Algebra
 
+// TODO: Consider moving to the `Algebra` module.
 extension ClosedRange: Zero where Bound: Zero {
 
     public static var zero: ClosedRange {

Sources/DataStructures/RangeProtocol.swift → Sources/DataStructures/Extras/RangeProtocol.swift

@@ -6,6 +6,8 @@
 //
 
 /// Unifying interface for `Range` and `ClosedRange` types.
+///
+/// TODO: Consider using Standard Library's `RangeExpression` protocol instead.
 public protocol RangeProtocol {
     associatedtype Bound: Comparable
     var lowerBound: Bound { get }

Sources/DataStructures/SequenceExtensions.swift → Sources/DataStructures/Extras/SequenceExtensions.swift

@@ -5,6 +5,7 @@
 //  Created by James Bean on 6/29/18.
 //
 
+// TODO: Consider moving to the `Algorithms` module.
 extension Sequence {
 
     /// All of the values which are the least or greatest given the `areInIncreasingOrder` closure.

Sources/DataStructures/SetExtensions.swift → Sources/DataStructures/Extras/SetExtensions.swift

@@ -5,6 +5,7 @@
 //  Created by James Bean on 6/29/18.
 //
 
+// TODO: Consider moving to the `Algorithms` module.
 extension Set {
 
     public func inserting(_ element: Element) -> Set {