Remove initialize and make finalize optional

[nhz2]

Fixes #70 by initializing on construction and finalizing in a finalizer.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.64%. Comparing base (e7edfed) to head (fdb6ec9).

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
- Coverage   61.55%   60.64%   -0.92%     
==========================================
  Files           5        5              
  Lines         372      371       -1     
==========================================
- Hits          229      225       -4     
- Misses        143      146       +3     

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[nhz2]

ZSTD_createCStream_advanced is part of the experimental API though according to facebook/zstd#1734 these functions should not change much in the future.

I am using it because the regular ZSTD_createCStream leads to Julia not running the finalizer before being killed by OOM.

[mkitti]

We should not rely on the GC to finalize external resources.

[nhz2]

We should not rely on the GC to finalize external resources.

Should I try and support both explicit-free when we know the Codec cannot escape and automatic-free for more complex situations?

[nhz2]

atomics require 1.7