Spaceship operator

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• Spaceship operator <=> returns -1, 0, or 1

examples/other/spaceship_operator.cr

puts 3 <=> 3 # 0
puts 3 <=> 4 # -1
puts 4 <=> 3 # 1

puts "a" <=> "a" # 0
puts "a" <=> "b" # -1
puts "b" <=> "a" # 1