RFR 8198962: Move HeapScope/NativeScope out of public API

[Maurizio Cimadamore]

Hi,
this change moves HeapScope/NativeScope out of the public API. The Scope 
interface is more than enough for clients - there's no need to expose 
the guts of how scopes are implemented. Instead, clients can easily 
create the desired scope using two factory methods in the Scope interface:

Scope::newNativeScope

Scope::newHeapScope

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8198962/

Cheers
Maurizio