[foreign-memaccess] RFR 8236267: Cleanup support for layouts with undefined size

[Maurizio Cimadamore]

Hi,
I decided to go ahead and create a separate issue for dealing with 
unbounded sequence layouts, since it seems like the API has some aspects 
which makes it difficult to deal with them:

* it is not possible to establish whether a layout has a size, and hence 
whether e.g. passing it to MemorySegment::allocateNative will blow up
* there's no way, given a layout which contains one or more unbound 
sequences to replace them with bounded ones - w/o recreating everything 
from scratch

This patch fixes these problems by:

* adding an hasSize() predicate to all layouts
* adding a way to 'map' a layout given a path - that is, create a new 
layout where one nested element has been replaced using a given unary 
operator

Separately, I've also added a way to 'select' a nested layout given a 
path (which is not strictly necessary, but seems a nice to have).

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8236267/

I've also updated javadoc here:

http://cr.openjdk.java.net/~mcimadamore/panama/memaccess_javadoc/jdk/incubator/foreign/package-summary.html

I've done a significant cleanup of the javadoc associated with the 
methods accepting layout paths (as well as their implementations), as 
there were a number of issues:

* the javadoc were not documenting that some path element selection 
could fail (e.g. if traversing a layout with no size)
* related, the newly created methods should be robust to the lack of 
size in a traversed layout (as they are more about selection than about 
precise offset computation)
* we were not clear (both in spec and impl) re. which PahElement are 
allowed on each of the path-accepting methods. For instance, passing 
PathElement.sequenceElement() to MemoryLayout::offset() makes no sense, 
as it does not select a single layout element with given offset. 
Similarly, the newly added methods have other restrictions which are 
documented (and tested)
* I've added more examples in the MemoryLayout toplevel javadoc, to show 
how layout paths can be used

Cheers
Maurizio