Add initial docs to crossbeam-skiplist

[caelunshun]

As per #503 (comment), I'm writing the docs for crossbeam-skiplist.

Checklist:

• Crate-level docs
• Docs for SkipMap
• Docs for SkipSet

I also have some thoughts about the API:

• Entry's name is misleading; it's not the same as the standard library's map entry API. A name like Ref might be more suitable, to align with RefCell.
• set::Entry could implement Deref<Target=T> for convenience.
• I see the base SkipList struct is exposed, though I'm not sure if it belongs in the public API.

[ghost]

This is amazing work <3

Entry is a lot like the new linked list Cursor: https://doc.rust-lang.org/nightly/alloc/collections/linked_list/struct.Cursor.html

The idea behind SkipList is that it is a low-level API for people who want more control than what SkipSet and SkipMap offer. I believe @Amanieu is using it.

[Amanieu]

SkipList is exposed because it is usable in no_std code. It does not rely on the thread-local Collector (since no_std doesn't have TLS).

[taiki-e]

Hi @caelunshun! Any updates on this? (It seems that there is still one checkbox left.)

If you don't have the bandwidth to work on the remaining things, I'd like to leave them to follow-up PR (by me or someone) and merge this to avoid conflicts.

BTW, this looks great. Thanks for working on this!

[taiki-e]

Thanks @caelunshun for working on this!

I added the remaining things to the checklist of the tracking issue (#635).

bors r+

[bors]

Build succeeded:

• ci