Tracker: rand_core v1.0

[dhardy]

This issue tracks breaking changes for rand_core v0.10 and other potential changes for v1.0.

Breaking changes scheduled for v0.10:

• Bump MSRV and Edition for rand_core #1668
• Move rand_core::impls::* to le #1667
• Move fn read_adapter out of trait TryRngCore #1669
• Move OsRng down to rand #1674

Problematic/blocked changes (more likely to hit v2.0 than v1.0):

• Can we merge BlockRng64 and BlockRng with impl<R: BlockRngCore<Item = u64>> RngCore for BlockRng<R>? No; firstly because Rust does not yet recognise that R: BlockRngCore<Item = u32> is distinct from R: BlockRngCore<Item = u64> and secondly because BlockRng64 has a half_used field.
• Can we replace usages of UnwrapMut with UnwrapErr<&mut Self>? It appears not since we don't have impl<R: TryRngCore> TryRngCore for &mut R (see next item).
• Can we have impl<R: RngCore> RngCore for &mut R? No; it conflicts with impl<R: RngCore + ?Sized> TryRngCore for R since Rust does not yet support overlapping blanket impls (e.g. specialisation).

Other possible changes:

• Rename Rng -> RngExt, RngCore -> Rng #1288
• rand_core: introduce Seed trait #1670
• Further block module revisions. Little motivation in my opinion.
• Tracker: proposed RngCore changes #1261

[dhardy]

Non-API-breaking changes:

• Remove doc section titled "Implementing SeedableRng for RNGs with large seeds". This is problematic only in that arrays of more than 32 elements do not currently implement Default. This limitation may be fixed soon; see Use const generics for array Default impl rust-lang/rust#61415.