Deprecate old-style constructor syntax

[som-snytt]

Ref #6325
Ref #3076

I wonder if there was a particular reason this wasn't done during the first round, besides that it was already a big delta.

Fixes scala/bug#11824

[eed3si9n]

Thanks @som-snytt!

I think it makes sense to make this on par with Dotty.
In 1d4d901 I just flipped settings.future warning into non-future warning, and didn't think about the constructor case.

[hrhino]

Woah, that was fast! I noticed this but I was hoping to have my last scrap of procedure syntax to hold on to until Dotty hits and I lose it all.

I realized that my own JUnit tests tend to exercise the worst of Scala's style:

@Test def `it oughta work` {
 ...
} 

which I think looks closer to the more scalatesty

it should "work" in {
...
}

and the (): Unit = is kinda just line noise. It's especially irritating because you can't use non-Unit methods as JUnit tests, so leaving off the type ascription is dangerous.

At least this makes secondary-constructor syntax, always a bit of a second-class syntax, more regular.

[som-snytt]

@hrhino agree, and that was ichoran's argument for proc syntax. However, nanotest is going to be so much better. Possible missed opportunity for nanu nanu test.

[hrhino]

"Neener, neener, I removed your procedure syntax"-test.

I'm always glad to find out I'm in agreement with Ichoran.