add deflateCopy

[folkertdev]

closes #68

[codecov]

Codecov Report

Attention: Patch coverage is 96.46570% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 88.22%. Comparing base (d98d1e3) to head (5f70bbf).

Files	Patch %	Lines
zlib-rs/src/deflate.rs	98.09%	5 Missing ⚠️
libz-rs-sys/src/lib.rs	81.81%	4 Missing ⚠️
zlib-rs/src/inflate.rs	94.44%	4 Missing ⚠️
zlib-rs/src/allocate.rs	90.90%	2 Missing ⚠️
zlib-rs/src/deflate/window.rs	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   88.25%   88.22%   -0.04%     
==========================================
  Files          34       34              
  Lines        8555     8728     +173     
==========================================
+ Hits         7550     7700     +150     
- Misses       1005     1028      +23     

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