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

int64.go

@@ -24,6 +24,7 @@ package atomic
 
 import (
 	"encoding/json"
+	"strconv"
 	"sync/atomic"
 )
 
@@ -93,3 +94,9 @@ func (i *Int64) UnmarshalJSON(b []byte) error {
 	i.Store(v)
 	return nil
 }
+
+// String encodes the wrapped value as a string.
+func (i *Int64) String() string {
+	v := i.Load()
+	return strconv.FormatInt(int64(v), 10)
+}