Results 2 issues of Natsuki Yagi

Add `DontImplCloneOnSqlType` attribute. Because, if schema definitions are separated into other crate, implementing Clone for ExistingType will happen compile error. cf. https://github.com/adwhit/diesel-derive-enum/issues/93

I have separated schema.rs into different crates as workspace because it takes time for macro expansion of diesel. If I define an enum in the workspace that references that crate...