bug: `PrimitiveLiteral` and `Literal` should not be `Ord`.

[liurenjie1024]

In rust, Ord means total order, while PartiarOrd mean partital order. Currently we have derived Ord for both PrimitiveLiteral and Literal, which is incorrect. How could we compare Struct with Map, or Date with String? We should do following changes:

1. Remove PartialOrd, Ord for both Literal.
2. Remove Ord for PrimitiveLiteral. I think we should also remove PartialOrd for PrimitiveLiteral, which misses type information for decimal.
3. Implement PartialOrd for Datum.

[liurenjie1024]

I think this is blocking prs with filter related, such as #363 and #367

[liurenjie1024]

cc @sdd @marvinlanhenke @Xuanwo @Fokko