revamping the incremental collector

Some simplifications (not counting bytes, couting only slots visited;
no more 'gcfinnum'); more GC parameters; using vararg in 'lua_gc' to
set parameters in different GC modes

lgc.h

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.h,v 2.96 2017/04/11 18:41:09 roberto Exp roberto $
+** $Id: lgc.h,v 2.97 2017/05/04 13:32:01 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -25,14 +25,6 @@
 */
 
 
-
-/* how much to allocate before next GC step */
-#if !defined(GCSTEPSIZE)
-/* ~100 small strings */
-#define GCSTEPSIZE	(cast_int(100 * sizeof(TString)))
-#endif
-
-
 /*
 ** Possible states of the Garbage Collector
 */
@@ -125,6 +117,16 @@
 	check_exp(getage(o) == (f), (o)->marked ^= ((f)^(t)))
 
 
+/* Default Values for GC parameters */
+#define LUAI_GENMAJORMUL         100
+#define LUAI_GENMINORMUL         5
+
+#define LUAI_GCPAUSE    100     /* 100% */
+#define LUAI_GCMUL      10
+/* how much to allocate before next GC step (log2) */
+#define LUAI_GCSTEPSIZE 13      /* 8 KB */
+
+
 /*
 ** Does one step of collection when debt becomes positive. 'pre'/'pos'
 ** allows some adjustments to be done only when needed. macro