Replace \ with std::concat

[oddcoder]

We are normalizing the usage of std::concat execlusively for splitting strings into multiple lines.

[trent-reed]

Huh, I guess you're right that it doesn't matter; this test passes https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=76641f6e256cca381c96290baa88c655

I would have figured it would just escape the line ending, so therefore kept the extra spaces (kinda like this for C: https://gcc.godbolt.org/z/eevTq736h), but apparently it does not in this case (it skips characters until the next non-whitespace, apparently - or something like that).

I'm used to doing this in C:

static const char kConstant[] = "Testing using line "
                                "ending slashes to continue a string.";

Which is why I tend to prefer something similar in format like concat!, but at least it's better defined in rust to do what one would expect. Anyways, I still have a slight preference for concat! for multiline string literals, but it does seem that it's more a matter of preference in rust.

Thanks for doing this!