Guard clauses for NSRequiresConcreteImplementation

[kouky]

Use guard statements to exit early with NSRequiresConcreteImplementation(). This approach is more succinct, and has the added safety of guard requiring a transfer of program control.

[parkera]

@swift-ci please test

[kouky]

@parkera the CI failure is the same for PR #372

_swift_stdlib_unicode_strToLower' can not be used when making a shared object

I've been working off the swift-3.0-preview-1-branch as a base as master has build failures on OSX when running the latest swift toolchain for Xcode. Merging master into this PR branch might fix the issue.

Just before I do that, can you please run CI against current master? It would be helpful to have a green baseline.

[parkera]

Yah we're in a bad state with the CI right now. I think it's going to clear up soon.

[parkera]

@swift-ci please test

[kouky]

@parkera i've merged master into this branch, the base commit for this branch is too old. Can we please run CI against it again?

[parkera]

This should be good to go now

[parkera]

@swift-ci please test

[kouky]

@parkera awesome, my first PR merged 🎉

[parkera]

Congrats and thank you!