Move clippy into CI

[Pat-Lafon]

With #228 and previously #217, a couple of pr's have been accepted to address clippy suggestions. Given this, it seems reasonable to move clippy into the github actions workflow.

This pr:

• Adds an additional, parallel job to the workflow to run clippy across the whole workspace.
• Picks some low hanging ci performance fruit by using CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse and not statically building z3 twice(As I later found mentioned in Stop ignoring errors in CI. #207 (comment)).
• Fixes the last of clippy's suggestions in various build/test files.

Depending on the status of #207, it would be faster to run clippy without static-link-z3. Sometimes people also move cargo fmt into the ci.

[waywardmonkeys]

Can you rebase the commit for running clippy in parallel into the commit where you added the config to run clippy in CI?

[Pat-Lafon]

I have cleaned up the commit history.