Drop unsafe code from truncate_old_buffer()

[Shnatsel]

This has no measurable performance difference. flate-bench was too noisy, so I've used hyperfine to measure it:

hyperfine --min-runs=25 --warmup=3 'target/release/examples/flate -i enwiki-latest-all-titles-in-ns0.gz -o /dev/null gzip-decode'

This also does not regress rustc version compatibility because ptr::add has been added in 1.26 while slice::split_at_mut has been around since 1.0 and slice::copy_from_slice since 1.9

[codecov-io]

Codecov Report

Merging #32 into master will increase coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   91.42%   91.46%   +0.03%     
==========================================
  Files          14       14              
  Lines        1295     1301       +6     
==========================================
+ Hits         1184     1190       +6     
  Misses        111      111

Impacted Files	Coverage Δ
src/non_blocking/deflate/decode.rs	86.85% <0%> (+0.07%)	⬆️
src/deflate/encode.rs	96.62% <0%> (+0.02%)	⬆️
src/deflate/symbol.rs	93.06% <0%> (+0.02%)	⬆️
src/huffman.rs	94.02% <0%> (+0.09%)	⬆️

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[sile]

LGTM. Thank you!