Keep correct type for immediate operands in comparisons

When calling metamethods for things like 'a < 3.0', which generates
the opcode OP_LTI, the C register tells that the operand was
converted to an integer, so that it can be corrected to float when
calling a metamethod.

This commit also includes some other stuff:
- file 'onelua.c' added to the project
- opcode OP_PREPVARARG renamed to OP_VARARGPREP
- comparison opcodes rewritten through macros

lopnames.h

@@ -92,7 +92,7 @@ static const char *const opnames[] = {
   "SETLIST",
   "CLOSURE",
   "VARARG",
-  "PREPVARARG",
+  "VARARGPREP",
   "EXTRAARG",
   NULL
 };