fix: type signature
fix: examples_dir
delete customized `show' functions to fix homoiconicity

Squashed commit of the following:

commit 713cc04cc179085cd419be5808ceb4ba8c4e1d53
Author: HIRAI, Hajime <hirai.hajime@gmail.com>
Date:   Sat Jun 22 18:17:12 2013 +0900

    delete customized `show' for UnaryOp and BinaryOp

commit b4335cba7481eeba388cfae564ec0c7008e06f43
Author: HIRAI, Hajime <hirai.hajime@gmail.com>
Date:   Sat Jun 22 17:58:18 2013 +0900

    delete customized `show' for Number

commit 976cf97f2cfedf1976f83518c87b3bf2a8dd5f38
Author: HIRAI, Hajime <hirai.hajime@gmail.com>
Date:   Sat Jun 22 17:23:11 2013 +0900

    delete customized `show' for CaseWord

commit a390bcec2c4304305fd4601e0c7f5ca0a4d59930
Author: HIRAI, Hajime <hirai.hajime@gmail.com>
Date:   Sat Jun 22 17:16:37 2013 +0900

    class Keyword, to delete customized Show instances