[foreign-memaccess] RFR 8234337: Misc foreign memory acess API cleanups

[Maurizio Cimadamore]

Hi,
quoting from the JBS issues, there are a number of issues in the API 
which require rectification:

* all views should use uniform naming (e.g. asXYZ)
* projections should use different naming (e.g. toByteBuffer)
* a projection from segment to byte array would be desirable
* segment factories should make it explicit as to whether allocation 
should be explicit
* segment pinning is mostly redundant now that we have acquire, and 
should be dropped (also, asPinned doesn't play nicely with clients using 
try with resources)
* the javadocs for MemoryLayout, MemorySegment, MemoryAddress and 
PathElement should be stricter about their immutability, sealedness and 
safety guarantees

This webrev addresses the issues above:

http://cr.openjdk.java.net/~mcimadamore/panama/8234337/

And an updated javadoc associated with these changes is available here:

http://cr.openjdk.java.net/~mcimadamore/panama/8234337_javadoc/

Main changes:

* all views (whose temporal bound is shared with original segment) are 
called asXYZ (asSlice, asReadOnly)
* asPinned/isPinned have been dropped
* asByteBuffer has been renamed to 'toByteBuffer', and a new 
'toByteArray' has been added (with a new test)
* MemorySegment::ofNative renamed to 'allocateNative'; 
MemorySegment::ofPath renamed to 'mapFromPath'
* misc javadoc improvements

I think that now, if you look at instance methods in MemorySegments, the 
API looks pretty tight and consistent naming-wise.

Cheers
Maurizio