Ongoing code maintenance

[Azriel Hoh]

• Update dependencies for both shred, shred-derive -- ran cargo upgrade.
• Cargo lints (cargo fix)
• Clippy lints (manual).

Notes:

• warning: needless fn main in doctest appears when running cargo clippy for:

  • src/dispatch/builder.rs:31:4
  • src/dispatch/builder.rs:65:4
  • src/dispatch/par_seq.rs:151:4

  But those are false positives -- cargo test fails without fn main().