strings: move Clone to stringslite

This help package like unique use Clone.

Change-Id: I073125b7ba0e3b87e9cb78c3fc0b5bada78ba4cd

src/internal/stringslite/strings.go

@@ -8,7 +8,10 @@
 // Tests for these functions are in the strings package.
 package stringslite
 
-import "internal/bytealg"
+import (
+	"internal/bytealg"
+	"unsafe"
+)
 
 func HasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
@@ -122,3 +125,12 @@ func CutSuffix(s, suffix string) (before string, found bool) {
 	}
 	return s[:len(s)-len(suffix)], true
 }
+
+func Clone(s string) string {
+	if len(s) == 0 {
+		return ""
+	}
+	b := make([]byte, len(s))
+	copy(b, s)
+	return unsafe.String(&b[0], len(b))
+}

src/strings/clone.go

@@ -5,7 +5,7 @@
 package strings
 
 import (
-	"unsafe"
+	"internal/stringslite"
 )
 
 // Clone returns a fresh copy of s.
@@ -19,10 +19,5 @@ import (
 // For strings of length zero the string "" will be returned
 // and no allocation is made.
 func Clone(s string) string {
-	if len(s) == 0 {
-		return ""
-	}
-	b := make([]byte, len(s))
-	copy(b, s)
-	return unsafe.String(&b[0], len(b))
+	return stringslite.Clone(s)
 }