Optimize OP codes when return single value

[coderzh]

Hi, when I profiling fib(40), the op calls show that:

    496740420  op_SetSlot_i32
    331160281  op_u32_LessThan_ss
    331160280  op_Call
    331160280  op_i32_Subtract_ss
    165580141  op_CopySlot_32     <--- extra op codes in new version
    165580141  op_If_r
    165580140  op_i32_Add_ss

Old wasm3 version, fib(40):

    496740420  op_SetSlot_i32
    331160281  op_u32_LessThan_ss
    331160280  op_Call
    331160280  op_i32_Subtract_ss
    165580141  op_If_r
    165580140  op_i32_Add_ss

I reviewed code then I found that it is because of the Multi-values support(#23)

The new version pushed all Return Values to the stack, then followed other Argnument Values, this cause more op_CopySlot_xx generated in ReturnValues function.

In the real world, most of functions return single value. If the count of return values less or equals than 1, can the stack keeps the same as the old version? The slot of the last return value also be the first argument value's.

Case1:

slot
0	return0	argument0
1		argument1

Case2:

slot
0	return0
1	return1	argument0
2		argument1

BTW: The pywasm3 tests failed, should apply this commit:
coderzh/pywasm3@7ca8225