feat: minor linting

[nyurik]

A few minor linting changes:

• simplify a few match statements
• use .is_empty() instead of creating a new vector just to see if there is nothing in it
• do not use std::u64 - use u64 keyword instead. Note that MAX is deprecated when used the old way.
• remove mut in the &mut self for few functions that don't need it
• remove unneeded casting

[sylvestre]

Could you please update the commit message for something more explicit? Thanks

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[nyurik]

@sylvestre I was a bit lazy, fixed it now :)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)