Reduce GUARD_OBJ

[tompng]

Remove GUARD_OBJ for returned value.
Return value will remain on stack as long as the returned value is used.

GUARD_OBJ(retval, NewZeroWrapLimited(1, mx));
do_something(retval.real);
return retval.bigdecimal;

// ↓

retval = NewZeroWrapLimited(1, mx);
do_something(retval.real);
return retval.bigdecimal;

On the other hand, self is not always on the stack.

GUARD_OBJ(x, GetBDValueMust(self));

// ↓ this conversion is not GC safe

x = GetBDValueMust(self);

static VALUE BigDecimal_to_r(VALUE self){
  self is always on the stack
}

static VALUE BigDecimal_to_i(VALUE self) {
  self is NOT always on the stack
  because this function is used as `BigDecimal_to_i(CheckGetValue(c))`
}

[tompng]

I'll revisit after some C function (VpSqrt, BigDecimal_power, BigMath_s_exp, BigMath_s_log) are removed.
These functions calls BigDecimal_ function with non-self object which is not on the stack.
It means that BigDecimal_xxx(VALUE self) needs RB_GC_GUARD(self).