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.

ltests.h

@@ -23,11 +23,6 @@
 #define LUAI_ASSERT
 
 
-
-/* compiled with -O0, Lua uses a lot of C stack space... */
-#undef LUAI_MAXCSTACK
-#define LUAI_MAXCSTACK		400
-
 /* to avoid warnings, and to make sure value is really unused */
 #define UNUSED(x)       (x=0, (void)(x))