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/all.lua

@@ -127,8 +127,8 @@ else
   end
 
   Cstacklevel = function ()
-    local _, _, ncalls, nci = T.stacklevel()
-    return ncalls  + nci   -- number of free slots in the C stack
+    local _, _, ncalls = T.stacklevel()
+    return ncalls    -- number of C calls
   end
 end
 

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"

testes/errors.lua

@@ -530,10 +530,9 @@ local function testrep (init, rep, close, repc, finalresult)
   if (finalresult) then
     assert(res() == finalresult)
   end
-  s = init .. string.rep(rep, 10000)
-  local res, msg = load(s)   -- 10000 levels not ok
-  assert(not res and (string.find(msg, "too many registers") or
-                      string.find(msg, "stack overflow")))
+  s = init .. string.rep(rep, 500)
+  local res, msg = load(s)   -- 500 levels not ok
+  assert(not res and string.find(msg, "too many"))
 end
 
 testrep("local a; a", ",a", "= 1", ",1")    -- multiple assignment