Implement concat_bytes!

[syvb]

This implements the unstable concat_bytes! macro, which has tracking issue #87555. It can be used like:

#![feature(concat_bytes)]

fn main() {
    assert_eq!(concat_bytes!(), &[]);
    assert_eq!(concat_bytes!(b'A', b"BC", [68, b'E', 70]), b"ABCDEF");
}

If strings or characters are used where byte strings or byte characters are required, it suggests adding a b prefix. If a number is used outside of an array it suggests arrayifying it. If a boolean is used it suggests replacing it with the numeric value of that number. Doubly nested arrays of bytes are disallowed.

[rust-highfive]

r? @matthewjasper

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[Fishrock123]

Just pointing out that the RFC contains a bit of a contradiction and this PR's tests don't appear to test that case: rust-lang/rfcs#2509 (comment)

[syvb]

@Fishrock123 That appears to have been an oversight when updating the RFC. I've added a test for it.

[jackh726]

Rerolling

r? rust-lang/compiler

[JohnCSimon]

Looks like this is ready for review
@rustbot label: -S-waiting-on-author +S-waiting-on-review

[Mark-Simulacrum]

Squashed commits, looks good to me, thanks!

@bors r+

[bors]

📌 Commit eb56693 has been approved by Mark-Simulacrum