Add CompareAndSwap and Swap, Deprecate CAS (#111)

Adds CompareAndSwap and Swap methods to String, Error, and Value,
implemented by making use of Value.CompareAndSwap and Value.Swap
added in Go 1.17.

Following that, add CompareAndSwap to all other types with "CAS" methods
and deprecate CAS in favor of CompareAndSwap, since that's the convention
the standard library chose for these in Go 1.19.

uintptr.go

@@ -66,7 +66,14 @@ func (i *Uintptr) Dec() uintptr {
 }
 
 // CAS is an atomic compare-and-swap.
+//
+// Deprecated: Use CompareAndSwap.
 func (i *Uintptr) CAS(old, new uintptr) (swapped bool) {
+	return i.CompareAndSwap(old, new)
+}
+
+// CompareAndSwap is an atomic compare-and-swap.
+func (i *Uintptr) CompareAndSwap(old, new uintptr) (swapped bool) {
 	return atomic.CompareAndSwapUintptr(&i.v, old, new)
 }