Add spec for File::Stat.setgid?
Add spec for File::Stat.setuid?
Add spec for File::Stat.socket?
Add spec for File::Stat.sticky?
File.sticky? spec: code refactoring
Add spec for Kernel#instance_variable_defined?
Core refactoring: follow rubyspec coding guidelines

Put utility class into a special module in order to avoid name clashes.
Merge remote-tracking branch 'upstream/master'
Merge branch 'master', remote-tracking branch 'upstream/master'
Merge branch 'master', remote branch 'upstream/master'
Merge remote branch 'upstream/master'
Merge remote branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge branch 'master', remote-tracking branch 'upstream/master'
Merge branch 'master', remote-tracking branch 'upstream/master'