RFR (14) 8235837: Memory access API refinements

[Maurizio Cimadamore]

Hi,
following the JEP 370 code review and CSR, certain comments that have 
not been addressed have been added to the tracker issue:

https://bugs.openjdk.java.net/browse/JDK-8235837

For further evaluation. After some discussion in the Panama channel (see 
[1] and [2]), the Panama team has come to the following conclusions:

* the MemoryAddress::offset(long) method should be renamed to 
MemoryAddress::addOffset(long)
* the MemorySegment::isAccessible() predicate should be dropped and 
replaced with the MemorySegment::ownerThread() accessor
* a new predicate should be added to MemoryLayout, namely 
MemoryLayout::hasSize() to allow clients to establish as to whether a 
layout contains some unsized parts
* a new method should be added to SequenceLayout, namely 
SequenceLayout::withElementCount, to create a new sequence layout with 
new element count
* 2 new helper methods should be added to MemoryLayout, namely:
    - MemoryLayout::map - to allow transformation of a layout into a new 
layout (by replacing the sub-layout at given path with a new layout)
    - MemoryLayout::select - to allow selection of a sub-layout given a 
layout path

Overall, we think this set of change strikes a good compromise between 
usability, readability and minimality.

The javadoc for the layout path-accepting methods (both old and new 
ones) has been significantly refactored and improved - and the toplevel 
javadoc section on layout paths has been enhanced to cover all the 
concepts. The implementation for these methods has also been 
consolidated greatly, which revealed subtle bugs in the old 
implementation which have now been rectified (new tests have been added).

Webrev is available here:

http://cr.openjdk.java.net/~mcimadamore/8235837/

Specdiff for changes here:

http://cr.openjdk.java.net/~mcimadamore/8235837_specdiff/overview-summary.html

A draft CSR for this issue is available here (I will finalize once the 
review converges on the API changes):

https://bugs.openjdk.java.net/browse/JDK-8236853

I plan to push this to JDK 14 under the late enhancement process [3].

Cheers
Maurizio

[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-December/006873.html
[2] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-December/006885.html
[3] - https://openjdk.java.net/jeps/3#Late-Enhancement-Request-Process