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

llimits.h

@@ -325,6 +325,8 @@ typedef l_uint32 Instruction;
 #define luai_numeq(a,b)         ((a)==(b))
 #define luai_numlt(a,b)         ((a)<(b))
 #define luai_numle(a,b)         ((a)<=(b))
+#define luai_numgt(a,b)         ((a)>(b))
+#define luai_numge(a,b)         ((a)>=(b))
 #define luai_numisnan(a)        (!luai_numeq((a), (a)))
 #endif