Add parallel compression

[vasi]

zstdseek is great, but slow. Let's use more cores!

• Adds a FrameSource abstraction, a function that returns a bunch of frames in order
• Adds a ConcurrentWriter interface, which adds WriteMany to add all frames from a FrameSource using concurrency
  • WriteMany takes variadic options, including: WithConcurrency to manually set concurrency level, and WithWriteCallback to run a callback each time a frame is written
• Adds pretty complete testing: Compares the results of regular Write vs WriteMany, and benchmarks them
• Modifies benchmark to just fake writing results, so we don't have to hold so much data in memory
• Uses WriteMany in zstdseek

Results of benchmarks on my machine:

$ go test -bench=Write -benchtime=5s
goos: linux
goarch: amd64
pkg: github.com/SaveTheRbtz/zstd-seekable-format-go/pkg
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkWrite/128-12         	 6892479	       820.2 ns/op	 156.06 MB/s
BenchmarkWrite/Parallel-128-12         	 1000000	      7606 ns/op	  16.83 MB/s
BenchmarkWrite/4096-12                 	 1000000	      9277 ns/op	 441.52 MB/s
BenchmarkWrite/Parallel-4096-12        	  581739	      9826 ns/op	 416.85 MB/s
BenchmarkWrite/16384-12                	  214653	     24568 ns/op	 666.88 MB/s
BenchmarkWrite/Parallel-16384-12       	  390426	     14231 ns/op	1151.31 MB/s
BenchmarkWrite/65536-12                	   60656	     85210 ns/op	 769.11 MB/s
BenchmarkWrite/Parallel-65536-12       	  134016	     41333 ns/op	1585.55 MB/s
BenchmarkWrite/1048576-12              	    3476	   1730455 ns/op	 605.95 MB/s
BenchmarkWrite/Parallel-1048576-12     	   10126	    568612 ns/op	1844.10 MB/s

In tests of zstdseek, I see speedups of anywhere between 2x and 5x. I'm not sure why I can't quite hit the 12x I'd expect.

[SaveTheRbtz]

Really nice performance improvement! Thank you for the contribution!

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 12 lines in your changes missing coverage. Please review.

Project coverage is 79.34%. Comparing base (301c94b) to head (4e1ed4e).
Report is 43 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/encoder.go	42.85%	7 Missing and 1 partial ⚠️
pkg/writer.go	93.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #167      +/-   ##
==========================================
+ Coverage   77.34%   79.34%   +2.00%     
==========================================
  Files           7        7              
  Lines         384      460      +76     
==========================================
+ Hits          297      365      +68     
- Misses         61       67       +6     
- Partials       26       28       +2     

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[vasi]

Tried to add a bunch of silly tests to make codecov happy :)

[SaveTheRbtz]

Thank you for your contribution!