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

lparser.c

@@ -817,7 +817,7 @@ static void constructor (LexState *ls, expdesc *t) {
 
 static void setvararg (FuncState *fs, int nparams) {
   fs->f->is_vararg = 1;
-  luaK_codeABC(fs, OP_PREPVARARG, nparams, 0, 0);
+  luaK_codeABC(fs, OP_VARARGPREP, nparams, 0, 0);
 }