[Integer protocols] Make .negate() the customization point (#4413)

The SignedArithmetic protocol provides default implementations for both
negate() and negated(), with negate() calling negated(). This is inconsistent
with the Arithmetic protocol, which provides implementations of the nonmutating
methods that call their mutating counterparts. This flips the implementations
of negate() and negated() so an implementer only has to focus on mutating
methods.

Author: natecook1000

stdlib/public/core/Integers.swift.gyb

@@ -159,12 +159,14 @@ public protocol SignedArithmetic : Arithmetic {
 extension SignedArithmetic {
   @_transparent
   public func negated() -> Self {
-    return Self().subtracting(self)
+    var result = self
+    result.negate()
+    return result
   }
 
   @_transparent
   public mutating func negate() {
-    self = negated()
+    self = Self().subtracting(self)
   }
 }