give deflate::State::sym_buf its own allocation

[folkertdev]

The zlib-ng implementation overlaps pending_buf and sym_buf in the same allocation. The end of pending_buf will eventually cross over the initial start of sym_buf. Hence we can't really make this safe.

Giving sym_buf its own allocation is a simple solution, and does use more memory. But I think the alternative of some custom abstraction over the combined pending_buf/sym_buf would be very fragile. I think this is the better solution for now.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (d3f7de1) 81.07% compared to head (d46ec3d) 81.29%.

Files	Patch %	Lines
zlib-rs/src/deflate/algorithm/fast.rs	0.00%	1 Missing ⚠️
zlib-rs/src/deflate/algorithm/huff.rs	0.00%	1 Missing ⚠️
zlib-rs/src/deflate/algorithm/medium.rs	0.00%	1 Missing ⚠️
zlib-rs/src/deflate/algorithm/rle.rs	0.00%	1 Missing ⚠️
zlib-rs/src/deflate/algorithm/slow.rs	0.00%	1 Missing ⚠️
zlib-rs/src/deflate/pending.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   81.07%   81.29%   +0.21%     
==========================================
  Files          28       28              
  Lines        6563     6569       +6     
==========================================
+ Hits         5321     5340      +19     
+ Misses       1242     1229      -13     

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