Remove Uniqued<Base>

This type was one of the prototypes for `OrderedSet` — it allowed us to customize the collection type used for element storage. (E.g., using `Deque` instead of `Array` to get efficient insertions at the front.) The original idea was that `OrderedSet` would simply be a typealias:

```swift
typealias OrderedSet<Element> = Uniqued<Array<Element>>
```

Unfortunately, while this concept is intellectually pleasing, in practice it just made the interface more complicated (and, in some cases, slower). Worse, it cannot be directly translated to an `OrderedDictionary`-style key-value mapping without major usability problems.

# Conflicts:
#	Package.swift

Package.swift

@@ -54,7 +54,6 @@ let package = Package(
     .library(name: "Collections", targets: ["Collections"]),
     .library(name: "DequeModule", targets: ["DequeModule"]),
     .library(name: "OrderedCollections", targets: ["OrderedCollections"]),
-    .library(name: "UniquedModule", targets: ["UniquedModule"]),
 
     .executable(name: "swift-collections-benchmark", targets: ["swift-collections-benchmark"]),
   ],
@@ -126,16 +125,6 @@ let package = Package(
       name: "OrderedCollectionsTests",
       dependencies: ["OrderedCollections", "CollectionsTestSupport"],
       swiftSettings: settings),
-
-    // Uniqued<Base>
-    .target(
-      name: "UniquedModule",
-      dependencies: ["DequeModule"],
-      swiftSettings: settings),
-    .testTarget(
-      name: "UniquedTests",
-      dependencies: ["UniquedModule", "CollectionsTestSupport"],
-      swiftSettings: settings),
   ],
   cxxLanguageStandard: .cxx1z
 )