Optimize decompression function

Prepare for release

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

Speed-Up extraction with concurrent decompression

A new release include feature to extract files in concurrent way if capable.
It will boost decompression performance for archive which has many files.

Basically uncompress working

First release of py7zr which is working basically for decompression.

There are some limitations.

• There is no option to control overwrite.

• There are unsupported compression algorithms which is not supported by python core.

  • BCJ2, PPMd