Decompress shrink more fields

[folkertdev]

a bunch more fields that can be smaller, unsigned, etc.

[codecov]

Codecov Report

Attention: Patch coverage is 97.97980% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libbz2-rs-sys/src/bzlib.rs	94.11%	1 Missing ⚠️
libbz2-rs-sys/src/decompress.rs	98.78%	1 Missing ⚠️

Files with missing lines	Coverage Δ
libbz2-rs-sys/src/bzlib.rs	92.62% <94.11%> (ø)
libbz2-rs-sys/src/decompress.rs	93.58% <98.78%> (-0.21%)	⬇️