[foreign] RFR 8213343: Update the public access mode API

[Jorn Vernee]

Hi,

While reading through the Pointer class, I noticed there is a method 
`isAccessibleFor(int)` which is used to check if the pointer is 
accessible for a certain access mode.

The problem is that the `int` value that's expected as an argument is 
defined as the constants MODE_R, MODE_W and MODE_RW in 
BoundedMemoryRegion [1]. This is in an internal package, so there will 
be no way for the user to use those constants.

The other problem is that having an `int` as the argument type gives no 
hint to what values can be used there. I would like to advocate for 
switching to an enum there, since then it's a simple case of typing in 
the enum name, hitting the auto-complete key and getting a list of 
available argument values.

The third problem is that there seems to be no public API point for 
setting the access mode of a Pointer. I can imagine that somebody might 
want to give out a read-only pointer to a client (and maybe even a 
write-only pointer).

I have a patch that attempts to solve all 3 problems;

1 & 2: I have created an AccessMode enum in j.f.memory that is used in 
place of the `int`s.
3: I have added 2 methods to Pointer, asReadOnly and asWriteOnly, to 
create a new pointer from the given one with either of the 2 access 
modes.

As an additional cleanup I've also removed the mode field from 
BoundedPointer, since it was unused, and made the other fields private 
instead of public. In order to do the latter, I added 
putLongBits/getLongBits methods to BoundedPointer, which replaces direct 
access to the pointer's region put/get methods. I also added a package 
declaration to ArrayTest since my IDE (thanks Maurizio ;P) was 
complaining about a duplicate class, since there was a test class with 
the same name in the jextract test directory.

Related bug: https://bugs.openjdk.java.net/browse/JDK-8213343
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/accessmode/webrev.00/

Please let me know what you think about this.

Thanks,
Jorn

[1] : 
http://hg.openjdk.java.net/panama/dev/file/cfee9bc4ebe4/src/java.base/share/classes/jdk/internal/foreign/memory/BoundedMemoryRegion.java#l33