Add Default, Display, and Debug trait bounds to PrimInt

[jturner314]

The following trait bounds would be useful to add to PrimInt on the next breaking release:

• Default
• Display
• Debug

See also #36 and #43. This is a tracking issue for adding these trait bounds.

[cuviper]

I've decided to limit the clutter of breaking change issues, so they will now be tracked collectively in #47. I'm closing this issue for now as postponed but we can still discuss its merits here as needed.