Add decorator @check_wrapper for all check functions

A new decorator `@check_wrapper` is to move most of the I/O functionalities,
such as printing and writing the result into a file, outside of each
check function so that each check can focus on the validation logic
itself by minimizing the impact from a requirement change in the output
format and so on.
To support this, a new class `Result` is also introduced to make it
clear what a check function is expected to return. As long as `Result`
class is returned, the decorator `@check_wrapper` handles the printing
them to stdout and files.

Author: takishida