Add field init shorthand example.

[chriskrycho]

Note: this should not be merged until 1.17 lands and the field init shorthand feature lands.

Though this isn't required for rust-lang/rust#37340, I'd call it a nice bonus to have it ready to go when it does.

[rust-highfive]

r? @steveklabnik

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[mdinger]

It's pretty nice you're trying to update an external repo other than just normal official targets for this. This seems fairly unusual.

[chriskrycho]

It should be increasingly common, since whether a change should make its way into Rust By Example is one of the questions under consideration in writing, accepting, and stabilizing RFCs now that RFC 1636 was merged! Some RFCs may even go so far as to require updating RbE before stabilizing the feature. This one didn't, but it's also an easy win, so here we are!

[steveklabnik]

Closing in favor of #837, which was first. Thanks though! ❤️