Annotate the most primitive type of boxed type

[kanghyojun]

A return type annotation of __nirum_serialize__ has to show the most primitive type of boxed type even if a boxed type boxes another boxed type.

For example:

boxed a (text);
boxed b (a);

It has to be represented in Python to:

class A:
    def __nirum_serialize__(self) -> str:
        ...

class B:
    def __nirum_serialize__(self) -> str:
        ...

[dahlia]

@admire93 Isn't this solved by the PR #5?

[kanghyojun]

@dahlia i think i reported this issue when i misunderstood about boxed type. Now boxed type shows appropriate annotation. so it have to be closed.