Relax feature flag for value's std_support

[tisonkun]

This closes #654 (comment).

While I agree @KodrAus's comment at #654 (comment), this is not a generally breaking down kv_x to kv + x.

But that value's std_support can work with kv + std. kv_std introduces an extra feature of value-bag/error, which is only used for:

    pub fn to_borrowed_error(&self) -> Option<&(dyn std::error::Error + 'static)> {
        self.inner.to_borrowed_error()
    }

above.

[KodrAus]

Thanks @tisonkun. I think this specific change is reasonable to make 👍