Optimize Sequence.uniquified() (#201)

* Improve performance of Sequence.uniquified()

* Use Element not Iterator.Element

* Use Set.insert instead of Dictionary.updateValue

* Update Sources/ArgumentParser/Utilities/SequenceExtensions.swift

got it - sorry about that!

Co-authored-by: Nate Cook <natecook@apple.com>

Co-authored-by: Nate Cook <natecook@apple.com>

Author: schlagelk

Sources/ArgumentParser/Utilities/SequenceExtensions.swift

@@ -9,14 +9,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-extension Sequence where Element: Equatable {
+extension Sequence where Element: Hashable {
   func uniquified() -> [Element] {
-    var sequence = Array<Element>()
-    for element in self {
-      if !sequence.contains(element) {
-        sequence.append(element)
-      }
-    }
-    return sequence
+    var seen = Set<Element>()
+    return self.filter { seen.insert($0).0 }
   }
 }