Misc test/benchmark updates, and Sync for Sender/Reciever

[Dr-Emann]

Fixes #8.
Fixes #26.
Relates to #4, since the size of the waker seems to have decreased (looks like the discriminant is able to fit in the niche for a reference).

• Cleanup benchmarks to avoid creation in every case
• Use const generics to move code out of macro
• Upgrade to criterion 0.5 from 0.3
• Implement Sync for Sender/Receiver

[faern]

Hi! Sorry for the very slow reply. To be frank, I think this slipped my mind primarily since it does so much. I like one PR to do one thing. Here you have basically four PRs.

Some of these things have already been done/should not be done:

• The upgrade to criterion 0.5 has already been done in 85fd895
• Implementing Sync for Sender is being done in Implement Sync for Sender #55.
• Implementing Sync for Receiver is unsound and should not be done. See Implement Sync for Sender + Receiver #26 (comment)
• The waker size test was disabled in 2ccf485, due to it inherently being unstable.

[faern]

Thanks for wanting to contribute! But I'll close this for the reasons stated in the previous comment. If there are things you still want to contribute I'm open to discuss those :) And feel free to submit more PRs, focusing on solving a single thing at a time if possible, please.