Revamp the Reconstruct/ReconstructUnchecked traits, and add a construct.

[copybara-service]

Revamp the Reconstruct/ReconstructUnchecked traits, and add a construct.

After this CL, the state of affairs is that there's a safe and unsafe way to reconstruct a value in-place (without running assignment operators):

Unsafe: ctor::reconstruct(bar, c);
Safe: *var = ctor::construct(c);

Where, in the unsafe version, var : Pin<&mut T>, and in the safe version,var: &mut T. This corresponds with the notion that &mut T is always safe to Rust-move to/from, but Pin<&mut T> has potentially difficult safety preconditions, which depend on the exact value in question.

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Reconstruct and ReconstructUnchecked were added during the design process for ctor, and don't make as much sense now.

Reconstruct was designed, initially, so that you could safely reconstruct types which are never potentially-overlapping. Before C++20, it would be safe to do this for any final type.

But as of C++20, the cat is completely out of the bag, and it is a property of values, not types. Even final types can be potentially-overlapping. So there is no completely safe interface dealing with Pin<&mut T>. (The completely safe interface uses &mut T, and it's called "rust assignment"! :])

So: it no longer makes sense as an unsafe trait. It should be removed from the trait definition.

That means that unsafe should likely go on the method itself. That's what the ReconstructUnchecked trait does. It's the moral equivalent of *p.into_inner_unchecked() = x, but where x is a lazily-constructed value.

So: this CL deletes Reconstruct, and for that matter removes ReconstructUnchecked and replaces it with a bare function.

Also, this adds a basic construct() function, which you need to use for safe code dealing with bare values, and makes it easier to document what reconstruct does. In effect, reconstruct is the pinful version of *p.as_mut().into_inner() = construct(c);, which works even if the value is not Rust-movable.