Rename Ptr<T> to Nonnull<T*> (#832)

Note that ptr.h also includes an enable_if change, to help avoid https://bugs.llvm.org/show_bug.cgi?id=51881

executable_semantics/ast/ast.h

@@ -22,7 +22,7 @@ struct AST {
   // Import directives.
   std::vector<LibraryName> imports;
   // The file's ordered declarations.
-  std::vector<Ptr<const Declaration>> declarations;
+  std::vector<Nonnull<const Declaration*>> declarations;
 };
 
 }  // namespace Carbon