[foreign-memaccess] RFR 8224040: Misc improvements to layout API

[Maurizio Cimadamore]

Hi,
this cleans up some of the edges of the API:

* better spec for Layout::bitsSize (which can throw on unbound 
sequences, thx Jorn for pointing that out!)
* replace LayoutPath::isBound with LayoutPath::dimensions, and fix javadoc
* remove Ednianness enum, and just use old good nio ByteOrder

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8224040/

Cheers
Maurizio