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Add support for async command handlers.

Fix issue with multiword optional args of non-bool type.

Propagate global args to all nested commands under `self.global_args`.

Support global args.

Add nested command support.

Prepare for postponed annotation evaluation, which will be the default

in Python 4.0 (see #2).

Add `show_defaults` to `set_help`.

Add set_sharg_map util.

Make `_root` not an abstract method.