A new MP3Navigator class is added, which determines timestamps for byte
positions by resyncing to a frame and then interpolating between known
points on either side. The known points start out as the first frame's
position in the file at timestamp 0, and EOF at the timestamp for
FFmpeg's file duration estimate. New buffered ranges are interpolated
between those two points, but also between the end of a prior range and
the start of the next.
Since MP3 can have variable bitrate without declaring it in the file
header, we have to allow buffered ranges to shift forward as new data
arrives to make room for underestimated durations. This is done for all
ranges following the first that has been appended to, keeping the start
of the current range consistent, so that subsequent seeks within that
range remain consistent.
Seeking is also implemented within the navigator to ensure that the
byte<->timestamp mapping is consistent and the buffered ranges begin
exactly where the seek landed.
