Back to a stackless implementation

A "with stack" implementation gains too little in performance to be
worth all the noise from C-stack overflows.

This commit is almost a sketch, to test performance. There are several
pending stuff:

- review control of C-stack overflow and error messages;
- what to do with setcstacklimit;
- review comments;
- review unroll of Lua calls.

lparser.c

@@ -489,12 +489,14 @@ static void adjust_assign (LexState *ls, int nvars, int nexps, expdesc *e) {
 }
 
 
-/*
-** Macros to limit the maximum recursion depth while parsing
-*/
-#define enterlevel(ls)	luaE_enterCcall((ls)->L)
+static void enterlevel (LexState *ls) {
+  lua_State *L = ls->L;
+  L->nCcalls++;
+  checklimit(ls->fs, getCcalls(L), LUAI_MAXCCALLS, "C levels");
+}
 
-#define leavelevel(ls)	luaE_exitCcall((ls)->L)
+
+#define leavelevel(ls) ((ls)->L->nCcalls--)
 
 
 /*