intro

• hash code is not Hash key-value data structure.
• hash code is usually a short identifier of an object.
• the short identifier is usually a numeric value, so it is faster.
• eql?() method compare with two objects' hash codes.
• hash() method return the object's hash code.
• better overwrite ==, hash, and eql? methods.
• Array#uniq, Hash#lookups compare with hash code.