New iden paradigm

[tyt2y3]

As discussed, Arc, vtable tricks are removed.

The biggest behaviour change is late binding to early binding. previously the Iden is only rendered when the AST is being serialized, now that we pre-render it as we store that into the AST.

For majority of usecases in SeaORM though, since they're 'static str anyway, there should be no difference.

It is still possible to have struct MyIden(String) with some dynamic behaviour.

@Huliiiiii

[tyt2y3]

there is no way to read this code diff other than going commit by commit. every commit is compilable.

but summary of changes:

1. change unquoted() signature (biggest visible breaking change)

2. change quoted() signature

fn quoted(&self) -> Cow<'static, str> {

3. removed prepare() from Iden trait (moved into builder backend)
4. DynIden is just a Cow, SeaRc is just a helper
5. impl IntoIden for String (most asked, I guess)

[Expurple]

I love this

[tyt2y3]

special thanks to @Huliiiiii for bringing forward the proposal!