Dazl

* Made dazl responsive to the `DAML_LEDGER_URL` environment variable so that if a URL is not specified in code, it is read from the environment instead.
* Factored out some typical dazl boilerplate into a new API, `dazl.run`.
* Changes to how dazl is configured. This breaks backwards compatibility in the `dazl.client.config` module, so if you are referring to any of those classes directly, there may be minor code changes required (this is not common).

Drop deprecated symbols.
Formally drop support for all DAML SDKs before v0.13.x.

* Add support for more DAML-LF types.
* (Experimental) add support for generating a scenario from a series of Ledger API commands.

* Add the ability for clients to subscribe to notifications on package changes.
* Miscellaneous fixes to type annotations, particularly around event subscription.
* Removed the deprecated `ledger_create` and `ledger_archive` events (please use `ledger_created` and `ledger_archived` instead).

* Support for DAML-LF 1.6 (enums and interned package IDs).
* Make package uploading "wait" until packages have been detected from the `PackageService`.

Add support for `CreateAndExerciseCommand` and `ExerciseByKeyCommand`.