0.21.3: Baby Lynx

This is the last release to support building with Swift 4.0 and Swift 4.1.

Breaking

• None.

Enhancements

• If New Build System is enabled on Xcode, the doc command does not need to
  use the clean action on xcodebuild.
  Norio Nomura

• Use 'as' bridging on Linux when using Swift 4.2.
  JP Simard

Bug Fixes

• None.