chore: fix clippy warnings and apply rustfmt

[nyurik]

No functional change. cargo clippy --all-targets --all-features -- -D warnings now passes and cargo fmt --check is clean, so follow-up changes can be reviewed against a warning-free baseline.

• Drop redundant return statements and use lines of extern crates, and needless borrows and into_iter() calls.
• Add a default ChunkBuffer::is_empty (clippy's len_without_is_empty); existing implementations keep compiling.
• Name the long types clippy flags as too complex: HeapItem in the merger, ChunkSortError and SortResult in the sorter.
• Tests: assert! instead of assert_eq! with booleans, and io::Error::other.
• Reformat with the repository's rustfmt.toml.