Allow slice concatenation without initial value.

[airblast-dev]

This is done by making use of core::mem::MaybeUninit and then transmuting the array, once all of the values are initialized.

The older syntax was left in for backwards compatibility purposes.

There is only a single line of unsafe code added as it was required for the change.
The safety of the unsafe block has been explained right above as a comment, its related documentation has been linked.

The related sections in documentation has been updated, and a testing case was added for the change that was made.

That being said, since the PR does introduce unsafe code, and it might not be desired to introduce unsafe code in the library, which I completely understand.

[airblast-dev]

It seems it fails due to the unused variable when using the old syntax.
Easy solution would be to change the first branch of the macro to:

([$init:expr; $T:ty]: $($s:expr),* $(,)?) => {{
       {let _ = $init;};
       $crate::_concat_slices!([$T]: $($s),*)
}};

We could either add an underscore to the test variable, and let clippy warn the users that they can remove the initializer, or completely silence it by changing the macro branch as shown above.

Just did some tests in my application that uses constcat. Resulting binary doesn't change either way, as the release binary was exactly the same (used diff, and cmp for comparisons).

I'm personally on the side of warning the users.

[rossmacarthur]

Thanks for the PR! I suspected there might be some way to not have to specify the initializer but I didn't think of this, so thanks :)

[airblast-dev]

Everything is ready now. I decided to also add a few more test cases just in case.