AnyHashableSendable ergonomics

[stephencelis]

While working with this type I noticed a few deficiencies:

• First, the initializer takes an opaque type rather than an existential, and while existentials are typically automatically opened, there are cases in which this initializer will fail, e.g.:

  let x: (any Hashable & Sendable)?
  x.map(AnyHashableSendable.init)  // 🛑

  We can fix this by updating the initializer to be an any already.

• Second, comparing an AnyHashableSendable with another AnyHashable fails because of the underlying type, but there is an underscored public protocol in the standard library we can take advantage of that is called when a hashable type is cast to AnyHashable, and if we return the base value then things like this start to work:

  AnyHashableSendable(1) == 1 as AnyHashable  // true

• Third, while probably not common in practice, we can box up certain literals into AnyHashableSendable automatically, so let's do it. I do not think it's possible to box array or dictionary literals, but we can cover the more common cases.

[stephencelis]

Now the one behavior change to consider here is there's no way to recover the sendability anymore:

let x: AnyHashableSendable
let y: AnyHashable = x
y.base as? AnyHashableSendable      // `nil` now, previously not
y.base as? any Hashable & Sendable  // 🛑

🛑 Marker protocol 'Sendable' cannot be used in a conditional cast