Bump `edition` to 2024; MSRV 1.85+; version to v3.0.0-pre

This commit is intended as the base commit for other proposed breaking changes, almost all of which will involve bumping the MSRV close to this (in fact if we want `const fn` support for `black_box` we need to go all the way to 1.86, this doesn't quite do that yet). It includes a few different changes, and I'm happy to split these up into separate PRs if that's helpful: - Updates CI config - sets `permissions` to read-only - sets `RUSTFLAGS` to `-D warnings` to deny warnings - uses `dtolnay/rust-toolchain` instead of the unmaintained `actions-rs/toolchain@v1` - uses `run` directly instead of the unmaintained `actions-rs/cargo@v1` - removes feature-specific tests because features have been removed(!) - adds additional `unsafe` code testing using `cargo careful` and `cargo miri` - tests both `dev` and `release` builds - Bumps version to `3.0.0-pre` to denote breaking changes - Removes obsolete `html_root_url` - Bumps edition to 2024; note this changed rustfmt rules and so rustfmt has also been applied with the new rules - Bumps MSRV to 1.85.0 and adds `rust-version` setting to `Cargo.toml` - Removes obsolete settings related to Travis CI (which has since been replaced by GitHub Actions) - Adds `.github` to `exclude` in `Cargo.toml` - Bumps `rand` from `0.8` to `0.9` (though `0.10` will be released soon) - Removes all features: with the MSRV bump, we no longer need any of these feature gates as all features will be available (the `std` feature didn't actually use anything from `std` and is thus pointless) - Makes `core::hint::black_box` the only value barrier/fence strategy for `Choice`: despite the scary documentation, it should be superior to the previous `core::ptr::read_volatile` approach, at least based on questions I have posed to various Rust compiler engineers, and is also `const fn` stable in Rust 1.86. In subsequent commits, I plan on attempting to add the `cmov` crate which provides `asm!`-backed equivalents of `ConditionalSelect` and `ConstantTimeEq`