Record construction

[Brian Goetz]

> One small thing: Could the contents of this constructor be lifted into
> JavaDoc?

The contents of the constructor under this model is surely less burdened 
by boilerplate, but will still have implementation details that would 
make for some strange specification reading.  So its not quite liftable 
directly.  But the question is still a fair one; how do we better 
document preconditions, without requiring the user say everything twice?

One possibility, as discussed, is to attach these as their own thing: 
"require x > 0", which is code that is narrowly targeted enough to lift.

Another is to raise the Preconditions library to more than just a 
library; let the Javadoc tool have some special treatment for, say, 
Precondition calls that unconditionally executed in the constructor, 
where they are boiled down into a documentary form.  This has the usual 
problem of trying to distill intent from imperative code, but perhaps 
could be sufficiently constrained to work well enough for 95% of 
preconditions.

Kevin and I have been chatting offline about how some of the accidental 
problems with Preconditions can be improved with the compiler magic that 
was described in my recent document on JEP 303. I think that if we can 
clear some of these accidental hurdles, a standard Precondition library 
could be more practical, which might pave the way for some of this.  
(I'll let Kevin explain further.)

Either way, though, I think better support for DBC is an orthogonal 
feature; I don't think it significantly constrains our ability to 
deliver a records feature without it.