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

ljumptab.h

@@ -107,7 +107,7 @@ static void *disptab[NUM_OPCODES] = {
 &&L_OP_SETLIST,
 &&L_OP_CLOSURE,
 &&L_OP_VARARG,
-&&L_OP_PREPVARARG,
+&&L_OP_VARARGPREP,
 &&L_OP_EXTRAARG
 
 };