Add support for QueryValue

[tqwewe]

This PR is related to SeaQL/sea-query#146

With these changes, users can implement QueryValue on their own types and use them in all queries including inserts and selects.

Three derive macros have been added to help generate implementations:

• DeriveType
• DeriveDecode
• DeriveEncode

A lot of the source code from the sqlx::Type derive was copied, with attributes renamed to sea_orm, and a new attribute cast added to type_name = ..., which wraps the value in CAST(value as type) in queries.

See https://docs.rs/sqlx/0.5.7/sqlx/trait.Type.html#attributes for more info on the attributes.

Implementing an enum would look like this:

#[derive(Copy, Clone, Debug, PartialEq, Eq, DeriveType, DeriveEncode, DeriveDecode)]
#[sea_orm(type_name = "plan", cast)]
#[sea_orm(rename_all = "lowercase")]
pub enum Plan {
    Free,
    Basic,
    Pro,
}

---

Another example for a custom email domain type in postgres:

CREATE DOMAIN email AS citext CHECK (
  VALUE ~ '^[a-zA-Z0-9.!#$%&''*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$'
);

#[derive(Clone, Debug, PartialEq, Eq, DeriveType, DeriveEncode, DeriveDecode)]
#[sea_orm(type_name = "citext")]
pub struct Email(pub String);

[tyt2y3]

It really seems cool! At least for the API it offers.

Why the cast attribute is needed? In what cases where a cast is not needed?

[tqwewe]

Why the cast attribute is needed? In what cases where a cast is not needed?

The cast will wrap your type in a sql cast. In the case of the plan example, CAST('free' as plan).
I think cast is only needed for CREATE TYPE, but CREATE DOMAIN does not require a cast.

[tqwewe]

With the new issue made #252, I wil close this PR for now. If needed it can be reopened.