Code-generate a `waitFor` method for each waiter

[jbelkins]

For each waiter, code-generate a public waitFor...() method that provides the entry point to waiter functionality.

The waiter method should be code generated as described in the Swift Waiters design doc here.

Requirements:

• Generate the method, configuring reusable types as needed, and using the Acceptors generated in #448.
• Add tests as required to ensure that methods are generated correctly.

When complete, this work should merge to a Waiters project branch while awaiting the completion of the rest of the Waiters project.