Reduce mandatory core extensions for internal use only

[PikachuEXE]

I remember when I was creating #6, I saw a number of core extensions just for convenience for internal use. It's not a good practice to use such approach. (I guess it's Principle of least astonishment

Some articles I read that make me start this issue:

• http://solnic.eu/2015/06/06/cutting-corners-or-why-rails-may-kill-ruby.html
• http://hawkins.io/2015/05/the-ruby-community-the-next-version/