This can be used to model durations in a user-friendly way, while
allowing coercion into the desired/required time unit at the usage site.

There is now a default requirement that properties exposed by the APIs
and that have a duration-like name (ending in `duration`, `period`,
`timeout` or `ttl`) have a type of `@aws-cdk/cdk.Duration` and do not
contain a unit suffix (`Days`, `Millis`, `Seconds`, `Sec`, ...).

BREAKING CHANGE: Properties throughout the AWS Construct Libraries that
                 represent lengths of time have been re-typed to be
                 `@aws-cdk/cdk.Duration` instead of `number`, and were
                 renamed to exclude any unit indication.

1. Dropped the free-floating toSeconds function
2. Added test for unresolved tokens in constructor
3. Made .toString() return a breaking token.