url should be an alias of JSON-LD's @id keyword

Hello,

Happy to see this new CG fully dedicated to schema.org, congratulations!

I hope I don't throw to heavy of a ball into the game, but I didn't find
this issue already clearly addressed.

Copying from github issue which I just created
https://github.com/schemaorg/schemaorg/issues/410

<blockquote>
continuing discussion with @gkellogg from #239

grep '"@id":' schemaorg/data/examples.txt

```shell
 "@id": "http://www.ncbi.nlm.nih.gov/mesh/D02.078.370.141.450",
    "@id": "http://www.ncbi.nlm.nih.gov/mesh/C18.452.394.750.149",
  "@id": "#product",
  "@id": "#product",
  "@id": "#model",
          "@id": "284708449",
          "@id": "5b23b738-bb64-4829-9296-5bcb59bb0d2d",
    "@id": "http://mlb.com/ws2013g1",
  "@id": "http://mlb.com/ws2013g1",
  "@id": "http://mlb.com/ws2013g2",
```

just few occurrences of @id from commits by @danbri but those on Product
examples suggest @mfhepp
https://github.com/schemaorg/schemaorg/blame/sdo-gozer/data/examples.txt

while ImageObject and thumbnail have their own peculiarities, I really
have hard time to believe that in general schema.org doesn't actually
use "url" as "@id"

```shell
grep '"url":' schemaorg/data/examples.txt |wc
     50     100    2311
```

also mentioned on
https://github.com/schemaorg/schemaorg/wiki/JsonLd#improvements

* discuss whether url should be an alias of JSON-LD's @id keyword
</blockquote>

Cheers!