refactor datum from Option<ScalarImpl> to a struct

[BugenZhao]

Currently, we use Option<ScalarImpl> in RisingWave to represent a nullable value. Using type alias for Datum indeed caused a lot of problems for us. For example, people would easily get confused when they see Option<Datum> or Option<Option<ScalarImpl>>, where the outer option means existence and the inner option means null.

We should refactor Datum to a struct, e.g. struct Datum(pub Option<ScalarImpl>).

Note this issue requires significant refactor work. Please contact maintainers before proceeding.

https://github.com/singularity-data/risingwave-dev/blob/7c6fb9b42846d9e0414d420e2e21df327352251a/rust/common/src/types/mod.rs#L276-L277

https://github.com/singularity-data/risingwave-dev/blob/7c6fb9b42846d9e0414d420e2e21df327352251a/rust/common/src/types/mod.rs#L303-L316

[fuyufjh]

What's the benefit?

[BugenZhao]

What's the benefit?

There're lots of stuff related to datum ser/de implemented as standalone functions, since we cannot implement methods on a type alias Option<ScalarImpl>.

https://github.com/singularity-data/risingwave-dev/blob/ed6330a950732d83a2b348e087b9f6933c8153b5/rust/common/src/types/mod.rs#L355-L366

[fuyufjh]

However, as we discussed offline just now, this will cause plenty of changes.

BTW, functions like serialize_datum_into also looks okay to me :D