Tracking issue: rust-lang#44434

Improve last commit of rust_lang#75644

also add a new SAFETY comment and simplify/remove a closure

The "Alphabetic" property in Unicode 14 grew too big for the bitset
representation, panicking "cannot pack 264 into 8 bits". However, we
were already choosing the skiplist for that anyway, so this doesn't need
to be a hard failure. That panic is now a returned `Err`, and then in
`emit_codepoints` we automatically defer to skiplist.

Among other changes, documents whether allocations are necessary
to complete the type conversion.

Part of rust-lang#51430

Co-authored-by: Giacomo Stevanato <[email protected]>

Co-authored-by: Joshua Nelson <[email protected]>

Co-authored-by: Josh Triplett <[email protected]>
Co-authored-by: Laurențiu Nicola <[email protected]>

Co-authored-by: Laurențiu Nicola <[email protected]>

Add 'core::array::from_fn' and 'core::array::try_from_fn'

These auxiliary methods fill uninitialized arrays in a safe way and are particularly useful for elements that don't implement `Default`.

```rust
// Foo doesn't implement Default
struct Foo(usize);

let _array = core::array::from_fn::<_, _, 2>(|idx| Foo(idx));
```

Different from `FromIterator`, it is guaranteed that the array will be fully filled and no error regarding uninitialized state will be throw. In certain scenarios, however, the creation of an **element** can fail and that is why the `try_from_fn` function is also provided.

```rust
#[derive(Debug, PartialEq)]
enum SomeError {
    Foo,
}

let array = core::array::try_from_fn(|i| Ok::<_, SomeError>(i));
assert_eq!(array, Ok([0, 1, 2, 3, 4]));

let another_array = core::array::try_from_fn(|_| Err(SomeError::Foo));
assert_eq!(another_array, Err(SomeError::Foo));
 ```

stack overflow handler specific openbsd change.

std: Stabilize command_access

Tracking issue: rust-lang#44434 (not yet closed but the FCP is done so that should be soon).

Update to Unicode 14.0

The Unicode Standard [announced Version 14.0](https://home.unicode.org/announcing-the-unicode-standard-version-14-0/) on September 14, 2021, and this pull request updates the generated tables in `core` accordingly.

This did require a little prep-work in `unicode-table-generator`. First, rust-lang#81358 had modified the generated file instead of the tool, so that change is now reflected in the tool as well. Next, I found that the "Alphabetic" property in version 14 was panicking when generating a bitset, "cannot pack 264 into 8 bits". We've been using the skiplist for that anyway, so I changed this to fail gracefully. Finally, I confirmed that the tool still created the exact same tables for 13 before moving to 14.

…ersions, r=m-ou-se

Add documentation to boxed conversions

Among other changes, documents whether allocations are necessary
to complete the type conversion.

Part of rust-lang#51430, supersedes rust-lang#89199

…ound-html-gen, r=notriddle

Fix invalid HTML generation for higher bounds

Considering this is a bug, I cherry-picked the commit from rust-lang#89676 so it's merged more quickly.

r? ``@notriddle``