stdlib: improve existential collection performance for -Osize.

Making sure that makeIterator is always inlined, enables devirutalization of the iterator calls.
Inlining was not done with -Osize which resulted in pretty bad performance when iterating over an existential collection.

Author: eeckstein

stdlib/public/core/ExistentialCollection.swift.gyb

@@ -413,6 +413,7 @@ internal final class _${Kind}Box<S : ${Kind}> : _Any${Kind}Box<S.Iterator.Elemen
 {
   internal typealias Element = S.Element
 
+  @inline(__always)
   @inlinable
   internal override func _makeIterator() -> AnyIterator<Element> {
     return AnyIterator(_base.makeIterator())
@@ -710,6 +711,7 @@ extension Any${Kind} {
 %   else:
   /// Returns an iterator over the elements of this collection.
 %   end
+  @inline(__always)
   @inlinable
   public func makeIterator() -> Iterator {
     return _box._makeIterator()