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

lopcodes.c

@@ -101,7 +101,7 @@ LUAI_DDEF const lu_byte luaP_opmodes[NUM_OPCODES] = {
  ,opmode(0, 1, 0, 0, iABC)		/* OP_SETLIST */
  ,opmode(0, 0, 0, 1, iABx)		/* OP_CLOSURE */
  ,opmode(1, 0, 0, 1, iABC)		/* OP_VARARG */
- ,opmode(0, 0, 0, 1, iABC)		/* OP_PREPVARARG */
+ ,opmode(0, 0, 0, 1, iABC)		/* OP_VARARGPREP */
  ,opmode(0, 0, 0, 0, iAx)		/* OP_EXTRAARG */
 };