Remove integer suffixes from Show implementation

[tbu-]

They just clutter the output, reading

[0i32, 1i32, 2i32, 3i32]

is really hard.

[rust-highfive]

r? @brson

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[huonw]

These suffixes were part of the design fwiw. (I'm not expressing an opinion on whether they should be removed.)

[tbu-]

It's what's written in the design, but it didn't have much discussion. We also don't distinguish between printing Vec, &[], or String, &str, this is just for debugging views of underlying values.

This also negatively affects tests: tbu-/collect-rs@18069a4#diff-8a5ac7b178f2c29d23992135dce5db83R969

[aturon]

FWIW, I'm hoping to post an RFC shortly establishing clearer conventions for fmt::Show and fmt::String. This may be best discussed there.

[tbu-]

It's a pretty clear win in readability, so this could be merged without an RFC (?). Still need to fix stds tests though.

[alexcrichton]

cc rust-lang/rfcs#565, we may wish to hold off on a decision here until a decision is reached on that RFC

[aturon]

FWIW, the RFC suggests making the change here. I do agree we should wait until that discussion is finished, though, to avoid unnecessary churn.

[alexcrichton]

Thanks again for the PR @tbu-! The relevant RFC has now been accepted, so this is good to go. I ended up implementing this as well in #21457 (sorry I forgot about this PR!), so I'm going to close this in favor of that to reduce merge conflicts.