Various cleanups

[bjorn3]

• Follow Rust naming conventions for function names
• Reduce visibility of items where possible
• Remove #[repr] where possible and remove a couple of unused fields exposed by this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
blocksort.rs	97.52% <100.00%> (ø)
bzip2.rs	82.68% <ø> (ø)
bzip2recover.rs	97.83% <100.00%> (ø)
bzlib.rs	88.35% <100.00%> (ø)
c2rust-lib.rs	98.21% <ø> (ø)
compress.rs	99.63% <100.00%> (ø)
decompress.rs	94.13% <100.00%> (ø)
huffman.rs	96.87% <100.00%> (ø)