Hide Interpreter in .cpp (#1036)

This improves encapsulation, and makes Interpreter lifetimes clearer (and shorter).

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/exec_program.cpp

@@ -41,7 +41,7 @@ void ExecProgram(Nonnull<Arena*> arena, AST ast, bool trace) {
     }
     llvm::outs() << "********** starting execution **********\n";
   }
-  int result = Interpreter(arena, trace).InterpProgram(ast);
+  int result = InterpProgram(ast, arena, trace);
   llvm::outs() << "result: " << result << "\n";
 }