Make concatenation of SubString{AnnotatedString} preserve annotations

[tecosaur]

SubStrings have been overlooked, and thanks to a few compiler quirks (relating to inlining and effect analysis), adding support for them is unfortunately a little more complicated than adding a || s isa SubString{<:AnnotatedString} clause thanks to the new generated runtime-checks.

To maintain the zero-overhead non-annotated code path, we need to implement a separate function _isannotated, which we also make use of to simplify the current join method.

[tecosaur]

I must admit, I can't see what's going on with CI here.

[tecosaur]

Just pushed the use of _isannotated to simplify lpad and rpad.

[tecosaur]

The CI failure seems unrelated:

make[1]: *** [stdlib/libLLVM_jll.release.image] Terminated: 15
Terminated: 15
🚨 Error: The command exited with status 143

[tecosaur]

Now with tests :)

[tecosaur]

Ok, what's going on with CI? It seems consistently on the fritz again.

[tecosaur]

I'd think this to be a pretty straightforward small improvement that could be quickly merged, is there anything holding this back?

[tecosaur]

It would be great if somebody with merge rights could spare the time to have a look at this PR.

[LilithHafner]

This construction

if annotated
    annotatedstring
else
    string
end

doesn't feel natural in the generic-function/multiple-dispatch paradigm, which makes me averse to touching this. However, that aversion does not block this PR as this PR simply improves the situation by shifting some existing hardcoded || statements into generic functions (in order fix some substring functionality)

[tecosaur]

I get that aversion. However, the reason why "if annotated" now pops up in a few places is because it's extended a status quo of "hardcoded String". I'm not sure what a more idiomatic-looking (and non-breaking) approach would look like, and nobody really suggested one during the original review of this feature.

I do recall Valentin (Churavy) suggesting that annotatedstring should be merged with string (which I'd personally be quite happy with), but IIRC one or two other people in triage veto'd that.