ci: test against more compilers, setup clippy and fix clippy lints

[cpcloud]

• Use cargo flags to prevent out-of-date Cargo.lock
• Ship the Cargo.lock file in the source distribution
• ci: add more variations to ci

[jimexist]

Thanks @cpcloud - i've incorporated some of your changes already, please rebase first?

[cpcloud]

@Jimexist done!