[Cra3z]

For sender adaptors that require both get_completion_signatures and transform_sender member functions (like starts_on and bulk):

• When an environment is provided (i.e., ex::get_completion_signatures<Sender, Env>()), it actually retrieves the completion signatures of the new sender resulting from transform_sender.
• The adaptor's own get_completion_signatures member is only invoked when no environment is provided (i.e., ex::get_completion_signatures<Sender>()).
• In this latter case, the behavior should be equivalent to passing an empty environment as the third argument to transform_sender to get a new sender, and then fetching the completion signatures of that new sender.

[coveralls]

coverage: 92.688%. remained the same — Cra3z:main into bemanproject:main