Speed up Set/Dictionary init from Sequence

Alternative to #5264

Author: Dave Abrahams

stdlib/public/core/HashedCollections.swift.gyb

@@ -762,7 +762,7 @@ public struct Set<Element : Hashable> :
   /// - Parameter sequence: The elements to use as members of the new set.
   public init<Source : Sequence>(_ sequence: Source)
     where Source.Iterator.Element == Element {
-    self.init()
+    self.init(minimumCapacity: sequence.underestimatedCount)
     if let s = sequence as? Set<Element> {
       // If this sequence is actually a native `Set`, then we can quickly
       // adopt its native storage and let COW handle uniquing only