Implement fmt.Stringer for atomic types (#76)

Add safe `String()` methods for atomic types that replicate the same
behavior as `fmt.Sprintf("%v", x.Load())` without the allocations.

As with json.Marshaler/Unmarshaler, we've omitted the `atomic.Value`
type for now.

Resolves #50

string_ext.go

@@ -20,6 +20,11 @@
 
 package atomic
 
+// String returns the wrapped value.
+func (s *String) String() string {
+	return s.Load()
+}
+
 // MarshalText encodes the wrapped string into a textual form.
 //
 // This makes it encodable as JSON, YAML, XML, and more.