Remove most trailing whitespace across cargo (particularly markdown)

[joshtriplett]

Regularly, people send in PRs, and their editor introduces unrelated
changes by removing trailing whitespace.

Proactively remove trailing whitespace from most markdown files and rust
source.

This leaves unmodified the trailing spaces in:

• Test snapshot output, because while we could represent the correct
  form, the snapshot testing tool won't use that format
• Generated files
• Manpages input, which we can handle in another pass

[rustbot]

r? @weihanglo

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The reviewer was selected based on:

• Owners of files modified in this PR: @epage, @weihanglo
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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[weihanglo]

Did this change because the trailing --- cargo after infostring? Can we remove only that trailing instead?

View changes since the review

[joshtriplett]

@weihanglo That would change what this test is testing; given the name of the test, I think it's trying to test whitespace here.

[epage]

Did this need to happen? rustfmt handles trailing whitespace so we shouldn't need to police it and it is annoying to work with source in this style. Every time I see code like this, I switch it away from it.

[weihanglo]

I think Json was looking from an editor's angle. I am fine with either, though I agree with Ed that the old style is way readable.