Reduce post-macro-expansion code in integer deserialize impls

[dtolnay]

This new implementation is less than half the code (+187, -389) and avoids creating work for the optimizer in quite a lot of places.

• Before: https://gist.github.com/rust-play/d5ea8b992cc58ef0968eaa158ad01902 + https://gist.github.com/rust-play/cdf5e8a4e8572d52df9706854edfc7fa

• After: https://gist.github.com/rust-play/de44c2234c4380418d95464424cef427

For example compare what happens when deserializing an i8 via visit_i8. Before:

            #[inline]
            fn visit_i8<E>(self, v: i8) -> Result<Self::Value, E>
            where
                E: Error,
            {
                match FromPrimitive::from_i8(v) {
                    Some(v) => Ok(v),
                    None => Err(Error::invalid_value(Unexpected::Signed(v as i64), &self)),
                }
            }
...
    impl FromPrimitive for i8 {
        #[inline]
        fn from_i8(n: i8) -> Option<Self> {
            if i8::min_value() as i64 <= n as i64 && n as i64 <= i8::max_value() as i64 {
                Some(n as i8)
            } else {
                None
            }
        }
        ...
    }

After:

            #[inline]
            fn visit_i8<E>(self, v: i8) -> Result<Self::Value, E>
            where
                E: Error,
            {
                Ok(v)
            }