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This PR fixes the wrong member name of SignalInfo introduced in #718. 

 - `num_samples` == `num_frames` * `num_channels`.

This is a part of PRs to add new "sox_io" backend #726, and depends on #718.

This PR adds `info` function to "sox_io" backend, which allows users to fetch some metadata of an audio file. 
At this moment, the information retrieved are;

 - Number of samples in the audio file
 - Sampling rate
 - Number of channels