* Feature-gate test that depends on non-default features
Makes `cargo check` work without extra flags.
* Don't set doc(html_root_url)
It is no longer recommended:
https://github.com/rust-lang/api-guidelines/pull/230
* Remove documentation URL from Cargo.toml
crates.io will link to the right version on docs.rs automatically.
* Ensure toolchains installed by actions-rs/toolchain are actually used
* Fix missing rustup component in check job
* Raise MSRV to 1.51
Older versions weren't actually working before.
* Only run clippy & rustfmt on stable toolchain
MSRV is checked in test-versions.
* Allow cargo doc to succeed without headers and multipart features
CI will still ensure that intra-doc links that rely on these are not broken.
