Fixes for compact printing

[nalimilan]

This is a follow-up from #16817. I think the second commit is uncontroversial.

The first one may need more design discussion. Setting :multiline=>false in showcompact sounds logical, but it's slightly annoying that it breaks the direct correspondence with the :compact=>true IOContext property. I don't know whether that's a problem or not. It could be good to rename either the method or the property, but we'd need to find a non-breaking solution.

[nalimilan]

If it's generally considered a good idea to check whether a flag is set to avoid setting it twice without reason, shouldn't we do this automatically under the hood from IOContext()? That would make this code much shorter.

[JeffBezanson]

It's worth noting that this is a bit of a corner case, since it's rare (and actually not a great practice) to define show for specific Types, and even rarer for a type to have a multi-line representation.

[nalimilan]

It's worth noting that this is a bit of a corner case, since it's rare (and actually not a great practice) to define show for specific Types, and even rarer for a type to have a multi-line representation.

Unfortunately, it's rare for types to define this (only Enum does in Base AFAIK), but it's more common to need representing these types, like for Nullable{Enum}. To be fully correct, all places where a type is printed should generally use showcompact, but since it's rare people will likely miss this until somebody tries with an enum. Ah, well...