Add support for the array concat operation in the dsl

[sintemal]

This adds support for the || operator on arrays. This feature was already requested in #1412

[Ten0]

implementation-wise this looks correct

I'm confident for merge if this is tested (either manually or by adding an auto-test) (once all the comments are solved of course)

[sintemal]

I think this should fix the documentation

[pksunkara]

Can you please repush to trigger the CI?

[sintemal]

One compiletest somehow fails, but i am not familiar enough with detailed typesystem to figure out why it does. Maybe you guys could take a look, if you can spot the bug. I could imagine that it has something to do with the nested type of Array<ST> and that __diesel_internal_SameResultAsInputin operators.rs doesn't support this...

[weiznich]

The error might be caused by an actually changed error message. Sometimes the error messages contains a number how many types implement a certain trait and that number changes if we add another impl (as that happens here). You can just run the compile tests with TRYBUILD=overwrite set to regenerate the changed files. Use the git diff to see what exactly changed and commit the changes if its something trivial as mentioned above.

[sintemal]

This actually seemed to be the problem. Thank you very much!