RFR 8143964: JShell API: convert query responses to Stream instead of List/Collection

[Robert Field]

Please review this deferred API review request.
Basically changing List<*> return to Stream<*> return for seven methods:
         Stream<Snippet> snippets()
         Stream<VarSnippet> variables()
         Stream<MethodSnippet> methods()
         Stream<TypeDeclSnippet> types()
         Stream<ImportSnippet> imports()
         Stream<Diag> diagnostics(Snippet snippet)
         Stream<String> unresolvedDependencies(DeclarationSnippet snippet)

Bug:
     https://bugs.openjdk.java.net/browse/JDK-8143964

Webrev:
     http://cr.openjdk.java.net/~rfield/8143964v0.webrev/

Spec:
http://cr.openjdk.java.net/~rfield/8143964v0.jshellAPI/jdk/jshell/JShell.html

Specdiff:
http://cr.openjdk.java.net/~rfield/8143964v0.specdiff/jdk/jshell/JShell.html

Thanks,
Robert