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

testes/db.lua

@@ -162,7 +162,7 @@ test([[for i,v in pairs{'a','b'} do
 end
 ]], {1,2,1,2,1,3})
 
-test([[for i=1,4 do a=1 end]], {1,1,1,1}, true)
+test([[for i=1,4 do a=1 end]], {1,1,1,1})
 
 
 do   -- testing line info/trace with large gaps in source