Remove hidden foo functions from doc examples; use Termination trait.

[frewsxcv]

Fixes #49233.

Easier to review with the white-space ignoring ?w=1 feature: https://github.com/rust-lang/rust/pull/49357/files?w=1

[rust-highfive]

r? @aidanhs

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[frewsxcv]

r? @rust-lang/docs

[GuillaumeGomez]

Are we sure we want to display the main function? Wouldn't it work if we didn't add the main function? My concern here is that it adds more content, which makes the examples longer and less pertinent.

[steveklabnik]

We do want to display main, IMO. The issue with this was people copy/pasting the docs and them being wrong; we hid the foo functions because they were so much noise, but the new main is not much noise at all.

[frewsxcv]

for posterity, asked this in irc last week, here's the logs: https://botbot.me/mozilla/rust-docs/2018-03-21/?msg=98071184&page=1

[GuillaumeGomez]

Ah I see the goal now. Well sounds good to me, so unless someone else finds a bug, r=me.

[frewsxcv]

@bors r=GuillaumeGomez rollup

[bors]

📌 Commit e9dcec0 has been approved by GuillaumeGomez

[kennytm]

@bors rollup- p=113

Given the diff amount I think rolling it up might be quite risky.

[bors]

⌛ Testing commit e9dcec0 with merge e5277c1...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: GuillaumeGomez
Pushing e5277c1 to master...