Move regexp implementation into its own folder.

src/regexp/regexp-stack.h

Index: src/regexp/regexp-stack.h
diff --git a/src/regexp-stack.h b/src/regexp/regexp-stack.h
similarity index 95%
rename from src/regexp-stack.h
rename to src/regexp/regexp-stack.h
index d18ce708d629b120fdecd510de0a206e62a77d42..210ec0cfe89a0c1f267d47da9cbac53fa8c77cf5 100644
--- a/src/regexp-stack.h
+++ b/src/regexp/regexp-stack.h
@@ -2,8 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef V8_REGEXP_STACK_H_
-#define V8_REGEXP_STACK_H_
+#ifndef V8_REGEXP_REGEXP_STACK_H_
+#define V8_REGEXP_REGEXP_STACK_H_
+
+#include "src/allocation.h"
+#include "src/globals.h"
+#include "src/isolate.h"
 
 namespace v8 {
 namespace internal {
@@ -122,4 +126,4 @@ class RegExpStack {
 
 }}  // namespace v8::internal
 
-#endif  // V8_REGEXP_STACK_H_
+#endif  // V8_REGEXP_REGEXP_STACK_H_