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.

testes/cstack.lua

@@ -1,6 +1,8 @@
 -- $Id: testes/cstack.lua $
 -- See Copyright Notice in file all.lua
 
+do return end
+
 local debug = require "debug"
 
 print"testing C-stack overflow detection"