* Make interface of Operator to `const Scope&`

`Paddle` manages Scope as programming language's scope.  It just a
thread-local stack of Scope. Top of that stack is current scope, the
bottom of that stack is all scopes' parent.

Invoking `create_var/get_var`  can `create/get` variable in current
scope. Invoking `enter_local_scope/leave_local_scope` can create or
destroy local scope.

A `scoped_function` will take a `function` as input. That function will
be invoked in a new local scope.