Use lookup tables safely

[Shnatsel]

Use regular indexing operations for lookup tables instead of get_unchecked(). This regresses performance by 0.7% - decoding enwiki-latest-all-titles-in-ns0.gz goes from 2,160s to 2,175s on my machine, as measured with hyperfine:
hyperfine -m 25 --warmup=3 'target/release/examples/flate -i enwiki-latest-all-titles-in-ns0.gz -o /dev/null gzip-decode'

Since the main selling point of a Rust implementation is safety, I don't think using unsafe for less than 1% speedup is justified.

[codecov-io]

Codecov Report

Merging #36 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #36   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files          14       14           
  Lines        1301     1301           
=======================================
  Hits         1190     1190           
  Misses        111      111

Impacted Files	Coverage Δ
src/deflate/symbol.rs	93.06% <100%> (ø)	⬆️
src/lz77/default.rs	89.1% <100%> (ø)	⬆️
src/huffman.rs	94.02% <100%> (ø)	⬆️

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[sile]

LGTM. Thanks!