String buffer refactoring, part 1.

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

src/lj_buf.c

@@ -0,0 +1,40 @@
+/*
+** Buffer handling.
+** Copyright (C) 2005-2013 Mike Pall. See Copyright Notice in luajit.h
+*/
+
+#include <stdio.h>
+
+#define lj_buf_c
+#define LUA_CORE
+
+#include "lj_obj.h"
+#include "lj_gc.h"
+#include "lj_err.h"
+#include "lj_buf.h"
+
+LJ_NOINLINE void lj_buf_grow(lua_State *L, SBuf *sb, MSize sz)
+{
+  MSize bsz = sb->sz * 2;
+  if (LJ_UNLIKELY(sz > LJ_MAX_MEM))
+    lj_err_mem(L);
+  if (bsz < LJ_MIN_SBUF) bsz = LJ_MIN_SBUF;
+  while (bsz < sz) bsz += bsz;
+  sb->buf = lj_mem_realloc(L, sb->buf, sb->sz, bsz);
+  sb->sz = bsz;
+}
+
+char *lj_buf_tmp(lua_State *L, MSize sz)
+{
+  return lj_buf_need(L, &G(L)->tmpbuf, sz);
+}
+
+void lj_buf_shrink(lua_State *L, SBuf *sb)
+{
+  MSize sz = sb->sz;
+  if (sz > 2*LJ_MIN_SBUF) {
+    sb->buf = lj_mem_realloc(L, sb->buf, sz, (sz >> 1));
+    sb->sz = (sz >> 1);
+  }
+}
+