[stdlib] Dictionary, Set: Force-inline hidden Sequence/Collection customization points

This should eliminate a branch, which should probably lead to a tiny overall code size improvement, as well as a tiny performance boost.

Author: lorentey

stdlib/public/core/Dictionary.swift

@@ -1383,16 +1383,19 @@ extension Dictionary {
     }
 
     @inlinable
+    @inline(__always)
     public func _customContainsEquatableElement(_ element: Element) -> Bool? {
       return _variant.contains(element)
     }
 
     @inlinable
+    @inline(__always)
     public func _customIndexOfEquatableElement(_ element: Element) -> Index?? {
       return Optional(_variant.index(forKey: element))
     }
 
     @inlinable
+    @inline(__always)
     public func _customLastIndexOfEquatableElement(_ element: Element) -> Index?? {
       // The first and last elements are the same because each element is unique.
       return _customIndexOfEquatableElement(element)

stdlib/public/core/Set.swift

@@ -266,6 +266,7 @@ extension Set: Sequence {
   }
 
   @inlinable
+  @inline(__always)
   public func _customContainsEquatableElement(_ member: Element) -> Bool? {
     return contains(member)
   }
@@ -361,13 +362,15 @@ extension Set: Collection {
   }
 
   @inlinable
+  @inline(__always)
   public func _customIndexOfEquatableElement(
      _ member: Element
     ) -> Index?? {
     return Optional(firstIndex(of: member))
   }
 
   @inlinable
+  @inline(__always)
   public func _customLastIndexOfEquatableElement(
      _ member: Element
     ) -> Index?? {