String buffer refactoring, part 1.

Move string buffer handling to lj_buf.*.
Use common buffer resizing function.

src/lj_buf.h

@@ -0,0 +1,27 @@
+/*
+** Buffer handling.
+** Copyright (C) 2005-2013 Mike Pall. See Copyright Notice in luajit.h
+*/
+
+#ifndef _LJ_BUF_H
+#define _LJ_BUF_H
+
+#include "lj_obj.h"
+
+/* Resizable string buffers. Struct definition in lj_obj.h. */
+LJ_FUNC char *lj_buf_tmp(lua_State *L, MSize sz);
+LJ_FUNC void lj_buf_grow(lua_State *L, SBuf *sb, MSize sz);
+LJ_FUNC void lj_buf_shrink(lua_State *L, SBuf *sb);
+
+#define lj_buf_init(sb)		((sb)->buf = NULL, (sb)->sz = 0)
+#define lj_buf_reset(sb)	((sb)->n = 0)
+#define lj_buf_free(g, sb)	lj_mem_free(g, (void *)(sb)->buf, (sb)->sz)
+
+static LJ_AINLINE char *lj_buf_need(lua_State *L, SBuf *sb, MSize sz)
+{
+  if (LJ_UNLIKELY(sz > sb->sz))
+    lj_buf_grow(L, sb, sz);
+  return sb->buf;
+}
+
+#endif