Tracking issue for the next breaking release

The next breaking release is likely to be v1.0.

Potential changes:

• Design of the block module. See #71
• Use SEED_LEN associated constant in the SeedableRng trait instead of the Seed associated type. Blocked on stabilization of generic_const_exprs.
• Migrate from Infallible to !. Blocked on stabilization of the never type. (Could be backwards-compatible?)
• Set Infallible as default type for TryRng::Error. Blocked on stabilization of associated_type_defaults.
• Introduce WordRng trait with blanket impls for TryRng (see below). Conflicts with the blanket impl of TryRng for T: DerefMut<Target = impl TryRng>. Blocked on https://github.com/rust-lang/rust/issues/20400, mutually exclusive traits and/or negative trait bounds.
• Remove UnwrapErr::re after support for reborrows is added to the language. See https://github.com/rust-lang/rfcs/issues/1403.
• Uninitialized buffer support. Blocked on https://github.com/rust-lang/rust/issues/117693.
• Remove (try_)next_u32/u64 methods? Suggested in https://github.com/rust-lang/rust/pull/157168

Rejected changes:

• Pass seeds by reference in SeedableRng instead of by value.
• Use "natural" seed types instead of byte arrays. See https://github.com/rust-random/rand/pull/1670.

This is a tracking issue rather than a self-contained code task; begin with the checklist and linked issue #71, then review each referenced Rust language blocker. It is done when the listed v1.0 changes are resolved or explicitly rejected and the release plan is updated.