Simplify PrimitiveLiteral

[ZENOTME]

solve #501

This is an initial PR for #501, I hope it can be a starting point to discuss what's going on if we simplify PrimitiveLiteral. For now, something still needs to be refined for this PR. e.g.

1. When we simplify PrimitiveLiteral, which means that in lots of cases when we use Datum, we should have a pattern like

 match (datum.data_type(),datum.literal()) {
  (...) =>
 }

It's tedious for me and I'm thinking about how to use this more clean and convenient. e.g fn as_date() -> i32 for Datum.