String buffer refactoring, part 1.

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

src/lj_load.c

@@ -15,6 +15,7 @@
 #include "lj_obj.h"
 #include "lj_gc.h"
 #include "lj_err.h"
+#include "lj_buf.h"
 #include "lj_str.h"
 #include "lj_func.h"
 #include "lj_frame.h"
@@ -54,7 +55,7 @@ LUA_API int lua_loadx(lua_State *L, lua_Reader reader, void *data,
   ls.rdata = data;
   ls.chunkarg = chunkname ? chunkname : "?";
   ls.mode = mode;
-  lj_str_initbuf(&ls.sb);
+  lj_buf_init(&ls.sb);
   status = lj_vm_cpcall(L, NULL, &ls, cpparser);
   lj_lex_cleanup(L, &ls);
   lj_gc_check(L);