[records] updates for Preview 9. hashCode

[Brian Goetz]

>
> This is almost OT but it's a question I was asked for twice,
> what is the rational to use the angle bracket instead of the parenthesis ?

I surveyed a variety of implementations of toString(), and picked what 
seemed most common, which was some variant of:

     Type[v=vVal, u=uVal]

Which was good enough.  Yes, we could bikeshed it, but I'm not 
particularly seeing a problem that needs to be fixed, and it doens't 
seem obvious to me that highlighting the record-ness of it is important 
at  toString-time?  It's the state that matters.