Expose the Pointer trait as public

[jeehoonkang]

This PR makes the following changes:

• Mark Pointer as public. This is necessary for the crates that implement data structures which expose Atomic-like API (e.g. store() and compare_and_swap()).

  I'm thinking of concurrent ART as a use case. It's basically the infinite array of Atomic<T>, where initially everything is just Atomic::null(). Basically, I'd like to expose Art::store<P: Pointer<T>>(index: usize, value: P).

• Add #![deny(missing_docs, warnings, missing_debug_implementations)], which is generally a good thing.

[ghost]

I don't feel comfortable with denying lints because future compiler updates could suddenly make our code stop compiling. See rust-lang/rust#47204 (comment) for a recent example of a new warning breaking existing crates. This can be especially frustrating for crates depending on crossbeam.

We should be fine with just #![warn(missing_docs, missing_debug_implementations)].

And if that is not enough, we can add RUSTFLAGS="-D warnings" to the Travis config. Better to break our CI builds than the whole ecosystem. :)

[glaebhoerl]

This can be especially frustrating for crates depending on crossbeam.

AFAIA cargo caps lints when building dependencies, so it would only affect crossbeam itself.

[jeehoonkang]

@stjepang I couldn't think of that :) I changed it to #![warn(missing_docs, missing_debug_implementations)].

[ghost]

Since we're publicly exposing Pointer, this might be a good time to reconsider the naming choices around Pointer::from_data and Pointer::into_data.

The word data is somewhat vague and could be mistaken for the actual data behind the pointer (of type T). Perhaps from_usize and into_usize would sound better? Any other ideas?

[jeehoonkang]

@stjepang I'm happy with from_usize and into_usize. I initially thought from_raw and into_raw, but they probably have a connotation with raw pointers.

[ghost]

@Vtec234, any thoughts on this rename?

[Vtec234]

Looks good, and I agree with the rename.

[jeehoonkang]

Based on two approvals, I'm merging it. Thanks!