Codegen for compact Entity format

[tyt2y3]

I think it's better for us to preserve the functionality to generate the current long Entity format, and can be switched on with a command line arguemnt.

• update codegen
• update sea-orm-cli to generate short format by default
• update docs

[billy1624]

So, at the end the codegen can produce codes of one of the formats? Let say, compact & expanded format.

[tyt2y3]

sea-orm-codegen 0.2.2 released

[tyt2y3]

I discovered a slightly strange behavior, is when there no relation, it will not derive DeriveRelation for the Relation enum.

// Filling.rs

#[derive(Copy, Clone, Debug, EnumIter)]
pub enum Relation {}

impl RelationTrait for Relation {
    fn def(&self) -> RelationDef {
        match self {
            _ => panic!("No RelationDef"),
        }
    }
}