Implement pooling of decompressed folder readers

[bodgit]

Assuming the common use case:

r, err := sevenzip.OpenReader("archive.7z")
if err != nil {        
        panic(err)
}
defer r.Close()   
                                                           
for _, f := range r.File {
        rc, err := f.Open()
        if err != nil {
                panic(err)
        }
        ...
        rc.Close()                                                                                                                                                                                                            
}

r.Close()

Comparing performance using the noopPool implementation (which doesn't pool so it's identical to the original behaviour) and the pool implementation:

$ benchstat old.txt new.txt 
name        old time/op  new time/op  delta
Bzip2-12    10.6ms ± 1%   2.3ms ± 1%  -78.54%  (p=0.008 n=5+5)
Copy-12     1.18ms ± 5%  1.19ms ± 2%     ~     (p=0.310 n=5+5)
Deflate-12  2.15ms ± 0%  1.22ms ± 2%  -43.43%  (p=0.008 n=5+5)
Delta-12    1.43ms ± 1%  1.16ms ± 0%  -19.03%  (p=0.008 n=5+5)
LZMA-12     15.2ms ± 2%   2.5ms ± 1%  -83.90%  (p=0.008 n=5+5)
LZMA2-12    7.88ms ± 1%  1.78ms ± 1%  -77.46%  (p=0.008 n=5+5)
BCJ2-12     3.73ms ± 1%  1.36ms ± 2%  -63.62%  (p=0.016 n=4+5)
Complex-12   35.4s ± 0%    0.2s ± 1%  -99.49%  (p=0.016 n=4+5)

The complex benchmark uses the 7-Zip/LZMA SDK archive which contains 633 files using a mix of compression methods. Reading this entire archive goes from taking 35.4 seconds to just 0.2 seconds!

The copy decompressor is the only one that doesn't improve (unsurprisingly) as it doesn't transform the bytes in any way.

Fixes #22