Breaking changes for v2.0

[cljoly]

A wishlist of breaking changes for the next major version:

• [ ] Change return types so that we can fix the cargo clippy pedantic warnings, like casting `i64` to `usize` may lose the sign of the value or casting `usize` to `u32` may truncate the value on targets with 64-bit wide pointers
• [ ] Introduce intermediary types so that we could write M::up(…).hook(…).down(…).hook(…)
• [ ] Remove deprecated items

Other things to decide on:

• [ ] Remove Eq implementation on M #45
• [ ] Remove PartialEq from our Error type?

I don’t know if a breaking version will ever be released though, there are costs to forcing users to migrate.