Redo generic handling

[oscartbeaumont]

I wonder if something like:

#[derive(Type)]
pub struct A {
   a: String
}
#[derive(Type)]
pub struct B {
   #[specta(inline)]
   b: Option<B>
}

should be export A = { b: { a: String } | null } instead of export A = { b: B | null }.

This would require a refactoring of the generic handling because currently the B comes from generics and is always a reference.

It would also be nice to add #[specta(inline_all)] which would opts.any_parent_inlined ? Type::inline : Type::reference for generics. This way.

Redo handling of opts. parent_inline. We should (in most cases) not pass opts directly to child impls because parent_inline is only valid for the type above the current one.

[oscartbeaumont]

This is blocked by limitations in stable Rust. If this playground (Rust issue) compiles on stable and meets Specta's MSRV/release cycle we can continue.

Potentially there is another way to make it work but this isn't important enough to try and make it work in a half-a**ed way.

[oscartbeaumont]

I think there is a better way to do this. Removing the generics param from the trait would be nice and it can probably be done by some internal inferences using the macro & Rust's proper type system.