Add finalizer to ZStream and make TranscodingStreams.finalize a noop

[nhz2]

Fixes #48 and fixes #88 by porting JuliaIO/CodecBzip2.jl#43 to this package.

According to https://www.zlib.net/manual.html#Basic

If zlib is used in a multi-threaded application, zalloc and zfree must be thread safe. In that case, zlib is thread-safe.

So, there shouldn't be any thread safety issues with the finalizer here.

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.84%. Comparing base (f29c86f) to head (f50e6f9).

Files with missing lines	Patch %	Lines
src/libz.jl	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   83.51%   84.84%   +1.33%     
==========================================
  Files           4        4              
  Lines         188      198      +10     
==========================================
+ Hits          157      168      +11     
+ Misses         31       30       -1     

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