Variable length arrays in Layouts

[Tobi Ajila]

We've been looking at how to support Variable Length Arrays in Layouts and
written up some of our initial thoughts at
http://j9java.github.io/panama-docs/VariableLengthArrays.

This only addresses the simplest case (fixed header repeating tail) but the
issues this raises need to be answered before the more complicated cases
can be addressed.

Please share your thoughts.