Converting `TableConstraint` struct enum variants into separated structs

Hi,

I need to implement traits describing for instance `CheckConstraint` and `UniqueConstraint` and so on for various crates, including sqlparser.

These objects are currently represented in the `TableConstraint` enum. I believe it would be better to split them into their own structs. 

I will open up a PR addressing this, but do let me know whether you agree that it is desirable.

Best,
Luca