[stdlib] Eliminate _AnyHashableBox._asSet() & ._asDictionary()

_canonicalBox can perform essentially the same job, so there is no reason to have these requirements.

(cherry picked from commit 2f4ad79)

Author: lorentey

stdlib/public/core/AnyHashable.swift

@@ -53,21 +53,12 @@ internal protocol _AnyHashableBox {
   var _base: Any { get }
   func _unbox<T: Hashable>() -> T?
   func _downCastConditional<T>(into result: UnsafeMutablePointer<T>) -> Bool
-
-  func _asSet() -> Set<AnyHashable>?
-  func _asDictionary() -> Dictionary<AnyHashable, AnyHashable>?
 }
 
 extension _AnyHashableBox {
   var _canonicalBox: _AnyHashableBox {
     return self
   }
-  func _asSet() -> Set<AnyHashable>? {
-    return nil
-  }
-  func _asDictionary() -> Dictionary<AnyHashable, AnyHashable>? {
-    return nil
-  }
 }
 
 @_fixed_layout // FIXME(sil-serialize-all)

stdlib/public/core/Dictionary.swift

@@ -1536,9 +1536,17 @@ internal struct _DictionaryAnyHashableBox<Key: Hashable, Value: Hashable>
     return _value
   }
 
+  internal var _canonicalBox: _AnyHashableBox {
+    return _DictionaryAnyHashableBox<AnyHashable, AnyHashable>(_canonical)
+  }
+
   internal func _isEqual(to other: _AnyHashableBox) -> Bool? {
-    guard let other = other._asDictionary() else { return nil }
-    return _canonical == other
+    guard
+      let other = other as? _DictionaryAnyHashableBox<AnyHashable, AnyHashable>
+    else {
+      return nil
+    }
+    return _canonical == other._value
   }
 
   internal var _hashValue: Int {
@@ -1560,10 +1568,6 @@ internal struct _DictionaryAnyHashableBox<Key: Hashable, Value: Hashable>
     result.initialize(to: value)
     return true
   }
-
-  internal func _asDictionary() -> Dictionary<AnyHashable, AnyHashable>? {
-    return _canonical
-  }
 }
 
 extension Dictionary: CustomStringConvertible, CustomDebugStringConvertible {

stdlib/public/core/NewtypeWrapper.swift

@@ -87,14 +87,6 @@ where Base: _SwiftNewtypeWrapper & Hashable, Base.RawValue: Hashable {
     }
     return false
   }
-
-  func _asSet() -> Set<AnyHashable>? {
-    return _canonicalBox._asSet()
-  }
-
-  func _asDictionary() -> Dictionary<AnyHashable, AnyHashable>? {
-    return _canonicalBox._asDictionary()
-  }
 }
 
 #if _runtime(_ObjC)

stdlib/public/core/Set.swift

@@ -519,9 +519,15 @@ internal struct _SetAnyHashableBox<Element: Hashable>: _AnyHashableBox {
     return _value
   }
 
+  internal var _canonicalBox: _AnyHashableBox {
+    return _SetAnyHashableBox<AnyHashable>(_canonical)
+  }
+
   internal func _isEqual(to other: _AnyHashableBox) -> Bool? {
-    guard let other = other._asSet() else { return nil }
-    return _canonical == other
+    guard let other = other as? _SetAnyHashableBox<AnyHashable> else {
+      return nil
+    }
+    return _canonical == other._value
   }
 
   internal var _hashValue: Int {
@@ -543,10 +549,6 @@ internal struct _SetAnyHashableBox<Element: Hashable>: _AnyHashableBox {
     result.initialize(to: value)
     return true
   }
-
-  internal func _asSet() -> Set<AnyHashable>? {
-    return _canonical
-  }
 }
 
 extension Set: SetAlgebra {