optimize AtomicError (#82)

This commit optimize AtomicError using atomic.Value. Benchmarks:

name               old time/op  new time/op  delta
AtomicError/Load-6   305ns ±11%    12ns ± 6%  -96.18%  (p=0.000 n=10+10)
AtomicError/Set-6   314ns ±16%    14ns ± 2%  -95.61%  (p=0.000 n=10+9)

core/errorx/atomicerror.go

@@ -1,21 +1,18 @@
 package errorx
 
-import "sync"
+import "sync/atomic"
 
 type AtomicError struct {
-	err  error
-	lock sync.Mutex
+	err atomic.Value // error
 }
 
 func (ae *AtomicError) Set(err error) {
-	ae.lock.Lock()
-	ae.err = err
-	ae.lock.Unlock()
+	ae.err.Store(err)
 }
 
 func (ae *AtomicError) Load() error {
-	ae.lock.Lock()
-	err := ae.err
-	ae.lock.Unlock()
-	return err
+	if v := ae.err.Load(); v != nil {
+		return v.(error)
+	}
+	return nil
 }