Make display of interval types more pretty

[Rachelint]

Which issue does this PR close?

Closes #5914

Rationale for this change

Make display of interval types more pretty.

What changes are included in this PR?

don't print the part in interval when it is zero.

Are there any user-facing changes?

[alamb]

Thanks @Rachelint -- this looks great. I think now that we have merged #6012 (thanks again BTW!) this PR could be merged / rebased and CI would pass

[Rachelint]

Thanks @Rachelint -- this looks great. I think now that we have merged #6012 (thanks again BTW!) this PR could be merged / rebased and CI would pass

Oh, forget to push yesterday... Have updated now.
Thanks for the review.

[alamb]

Thank you @Rachelint -- I think this looks great personally.

As @tustvold has signaled, I think this PR changes the semantics enough that we should treat it as a "public API" change and thus hold it for the next major release: #6016

[alamb]

You could probably avoid some repetition here if with a constant str. Something like

Suggested change

	if first_part {
	write!(
	f,
	"{}{}.{:09} secs",
	secs_sign,
	secs.abs(),
	nanoseconds.abs()
	)?;
	} else {
	write!(
	f,
	" {}{}.{:09} secs",
	secs_sign,
	secs.abs(),
	nanoseconds.abs()
	)?;
	}
	let prefix = if first_part { " " } else { "" }
	write!(
	f,
	"{prefix}{}{}.{:09} secs",
	secs_sign,
	secs.abs(),
	nanoseconds.abs()
	)

However that might be slightly slower now it has to check prefix. You could possibly even use a field like prefix: &'static str instead of bool first_part if you wanted to make it even more concise.

A similar comment applies to the other clauses

I don't think this is required, I just wanted to mention it

[alamb]

Marking as waiting on next major release. @Rachelint feel free to make changes as well; I am marking this as a draft until main opens for 53

[alamb]

Per discussion on #6050 I am going to merge this branch to the 53.0.0-dev feature branch, where we are collecting PRs with breaking changes before master opens

[Rachelint]

Per discussion on #6050 I am going to merge this branch to the 53.0.0-dev feature branch, where we are collecting PRs with breaking changes before master opens

Oh, sorry... forget to push the commit about eliminating the repeated codes, I make a new pr to make it.
Make it in #6080

[alamb]

Thank you @Rachelint