Allow reusing Tag instances.

The "bool" type is built-in.

Implement the "scan_stream" method that can read tags from any
input_stream object.  This requires a FLAC__IOCallbacks implementation
based on the input_stream API.

Merge common code.

Not using libFLAC++ because this library adds a lot of overhead due to
virtual method calls.  This new class library is zero-overhead.

This has been deprecated by the "embcue" playlist plugin.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

Don't calculate the song duration when the sample rate is 0 (division
by zero crash).