[stdlib] Fix print method syntax and add the optional binding

[dakeshi]

What's in this pull request?

• Fix print(:separator =) with print(:separator:terminator) syntax.
• First, last property of an array returns an optional type. So it needs to unwrap process.
  Add the optional binding for accessing optional first, last property

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• Add optional binding for using optional first, last property

[lattner]

LGTM, please don't change trailing whitespace on unrelated lines though, thanks!

[moiseev]

FWIW, terminator argument has a default value, which is exactly the \n.

[dakeshi]

Thank you for your feedback @lattner, @moiseev
I fixed it: b49b5c4

[gribozavr]

@swift-ci Please test and merge

[lattner]

Thanks!