Implement parser and version middleware.
Alias version() to middleware.
Migrate help to middleware.
Migrate unparsed argument type conversion.

So that it is specified as middleware.
Migrate defaults import to middleware.
Migrate env logic to middleware.
Migrate to merge and multiple middleware.

So that the logic for merging argument values into the stash is now
middleware. The multiple middleware is responsible for initializing
repeatable values that are not set as empty arrays and assigning them to
the stash for consistency.
Migrate required option to middleware.

Also migrate the default error listener to middleware.
Migrate the empty event handling to middleware.
Migrate the run event to middleware.
Migrate command execution to middleware.

Also configure sensible default middleware if use() has not been
invoked.
Fix help/defaults example.
Fix help/custom example.
Fix error examples.
Fix exec error handling.
Fix example executables.
Fix all tests.
Remove obsolete files.
Remove deprecated __actions property.

Also rename keys for the help and version options so they do not
conflict with the method definitions.
Move default help handler.