Invert public/private default

[rossberg]

This change makes private the default visibility for object fields. To make up for that, the new syntax is now a separate short-hand record form, where only <id> = <exp> syntax is allowed, and all fields are public. (In a future change I'd like to get rid of the new keyword for this, but that has various nasty grammar implications.)

The tests, especially the produce-exchange, show that this inversion often leads to more verbose code, but I think we agree that it's still worth it. I also noted that the PX code makes a hell lot of things public...

Also, some minor grammar tweaks.

[crusso]

That must have been fun. EDIT: I noticed Syntax.md doesn't document the current module syntax and inlined (some of) the changes needed).

Did you update the guide? EDIT: I didn't see it in the list of files...

[rossberg]

Guide updated, PTAL.

[crusso]

LGTM, thanks for fixing the slides too.