core: Clean up Debug formatting of meta::Visual

[zombiepigdragon]

This removes the often large and unreadable Debug formatting for the affected type (a raw [u8] dump for an embedded image file), and replaces the output for data with a manually-formatted type that only shows the data's size. An alternative would be using a wrapper type for Box<u8> that implements this manual debug format on its own, avoiding the possible maintenance burden this PR could have and being reusable for other blob-like data in Symphonia, but this was not chosen as it would likely become a breaking change for relatively little benefit.

[dedobbin]

I can imagine there are situations where you want to see the data, but you can always just print the .data member. So yeah makes sense and lgtm

[abique]

This PR looks good to me too.