RFR 8224134: Fix javadoc issues

[Maurizio Cimadamore]

There's an issue I uncovered in javadoc which is preventing javadoc for 
layout classes from being generated correctly:

https://bugs.openjdk.java.net/browse/JDK-8224052

While we're working on a fix, I think it's better to make the 
intermediate AbstractLayout type non-generic (which avoids the problem), 
and then sprinkle some covariant overrides.

I've also added some javadocs for equals/hashcode in various classes.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8224134/

Cheers
Maurizio