An attempt to make clippy and rustc complain less

[afeistel]

Submitting this as a draft PR, since there are somewhat many changes (I doubt anyone would be willing to review them all at once — I'll probably make separate PRs for each specific thing — just need some feedback on which of these are worth it; plus my fork currently has some changes which are just taste-based).

What I did (the important parts):

• #[repr(C, u8)] on enums. The way enums are currently handled seems to be potentially unsound.
• default-features (default_features is deprecated).
• #[non_exhaustive] on structs.
• PhantomDatafied unused tuple fields.
• Removed target_pointer_width = "128".
• Changed cfg(x86_64) to cfg(target_arch = "x86_64").
• Changed ScratchSpaceInner (keeping the same ABI as far as I understand) to have better user experience (fewer warnings) at the cost of slightly more complex definition and construction (should not impact runtime at all).
• Changed generated const names to _ where that made sense.
• Removed empty ///s — see #[sabi_trait] with #[warn(clippy::empty_docs)] #116.
• #[allow(clippy::needless_lifetimes)], #[allow(non_camel_case_types)] and #[allow(clippy::unnecessary_cast)] in proc-macros

[afeistel]

Sorry for a total mess of a PR, but I just actually don't know if this is maintained and what direction other people want this project moving.

[afeistel]

yes, I to forgot actually change the version in Cargo.toml and only wrote it in the changelog