Support dynamically sized types

[jeehoonkang]

It supersedes #209 for supporting dynamically sized types (DST) in Crossbeam. It's much cleaner and less intrusive than the previous attempts in that it doesn't introduce another Atomic type. Atomic<T> is still there without significant changes.

The key idea is (1) instead of requiring the condition on T: Sized, (2) requiring T: Pointable that means an object of type T can be pointed to by a single word. For instance, Atomic becomes: pub struct Atomic<T: ?Sized + Pointable> { ... }.

It is breaking the backward compatibility in that it (1) increases the minimum required Rust version to 1.36 (for MaybeUninit) and (2) now const fn Atomic::null() is Nightly only.

[jeehoonkang]

I think it's ready to be merged. If everyone's happy with it, I'll merge it 3 days later.

[jeehoonkang]

bors r+

[bors]

Build succeeded:

• dependencies
• rustfmt
• test (crossbeam, 1.36.0)
• test (crossbeam-channel, 1.36.0)
• test (crossbeam-channel, nightly)
• test (crossbeam-deque, 1.36.0)
• test (crossbeam-deque, nightly)
• test (crossbeam-epoch, 1.36.0)
• test (crossbeam-epoch, nightly)
• test (crossbeam, nightly)
• test (crossbeam-queue, 1.36.0)
• test (crossbeam-queue, nightly)
• test (crossbeam-skiplist, 1.36.0)
• test (crossbeam-skiplist, nightly)
• test (crossbeam-utils, 1.36.0)
• test (crossbeam-utils, nightly)