Relaxing the condition on pointer tags

[jeehoonkang]

I am porting @stjepang 's channel (https://github.com/stjepang/channel) from coco to crossbeam-epoch, and found out that channel uses pointer tags >= 8 on x86-64: https://github.com/stjepang/channel/blob/master/src/flavors/list.rs#L102

It turns out that coco allows tags >= 8, while crossbeam-epoch doesn't: https://github.com/stjepang/coco/blob/master/src/epoch/atomic.rs#L429 https://github.com/crossbeam-rs/crossbeam-epoch/blob/master/src/atomic.rs#L518

I am convinced that this relaxation of coco is actually more ergonomic, as evidenced by the channel implementation. Though it may be a little bit more unsafe (in terms of surprising the users). In this tradeoff, I would like to prefer the usability to the safety. What do you think?

Also, if we will decide to be strict on pointer tags in this matter, I think it would be great to expose atomic.rs's low_bits() in the public interface, for supporting the use case in channel.