Introduce Rollable, a trait for arbitrary types

[jakoschiko]

I lost hope that I can use a trait from another crate for that purpose:

• I removed support for quickcheck::Arbitrary recently, see Remove support for quickcheck #4.
• Maybe I'll add support for arbitrary::Arbitrary at some point, but it's far from optimal, see Find a way to integrate with arbitrary #6.

So let's introduce an own trait (+ derive macro). It could look like this:

pub trait Rollable {
    fn die() -> impl Die<Self>;
    fn uses_limit() -> bool;
}

#[derive(Rollable)]
struct MyStruct;

Remarks:

• Distinguishable name to prevent confusion with other crates.
• Rollable::die allows nice integration with exiting combinators.
• Rollable::uses_limit allows to implement a reliable strategy to respect Limit for the whole data structure.
• Recursive data structures are a big problem for arbitrary::Arbitrary. I can provide a better experience by splitting up Limit when recursing into the data structure.

[jakoschiko]

https://w-graj.net/posts/rust-derive-attribute-macros/