Expose preview and metadata

[philippeitis]

https://www.libraw.org/docs/API-datastruct.html#libraw_thumbnail_t

I've implemented functionality for this in my own fork (though it wouldn't be possible to merge it directly, since I've updated to the latest version of libraw for CR3 support), but the implementation could be reviewed.

https://www.libraw.org/docs/API-datastruct.html#libraw_imgother_t

Here, I think the impl would expose a number of getter methods, including for libraw_gps_info_t - possibly, in a more descriptive fashion than libraw does by default (gps timestamp might be hour, min, sec):

  typedef struct
  {
    float latitude[3];     /* Deg,min,sec */
    float longitude[3];    /* Deg,min,sec */
    float gpstimestamp[3]; /* Deg,min,sec */
    float altitude;
    char  altref, latref, longref, gpsstatus;
    char  gpsparsed;
  } libraw_gps_info_t;

I would be happy to implement this functionality as well, but wanted to ask if it would be useful to provide support for Rust libraries for timestamps or similar fields.