WDL Specification v1.1.1

This is a patch release to the WDL 1.1 specification - there are no new features or changes to functionality.

In addition, we are changing to a separate-branch-per-version model for the GitHub repository. The branch for WDL 1.1 will become the repository's default branch.

Changes

• Introduce the concept of "hidden types"; declare that None is of the hidden type Union.
• Add clarifications to several sections.
• Fix lots of typos.
• Organize the standard library hierarchically.
• Convert most code examples to test cases.

Details can be found in the pull request.