Split all backports individually :tada:
Fix tests & use rubyspecs
Fix to_h
Tweak advise
Fix File.write
Tweak Hash#select!
Improve meta programming
Fix File.expand_path
Tweak Float#round
Tweak {File|IO}.open
Tweak require_relative
Fix Proc#lambda?
Fix Symbol#=~
Tweak Complex#to_r
Tweak File.lchmod
Tweak Enumerator#rewind
Avoid warnings in 1.8.6
Tweak Array#permutation
Tweak Array#product
Fix array#rindex