Add == methods for IPv4 and IPv6

[asomers]

ActiveRecord requires these methods for seriaization. <=> is
insufficient

[smortex]

Is there a reason we can't rely on <=> to implement this?

Something like:

def ==(oth)
  (self <=> oth) == 0
end

Also, moving this in a common base class would avoid code duplication 🤔.

[asomers]

@smortex that would work in a statically typed language, but not in Ruby. Consider this:

irb(main):002:0> IPAddress.parse("1.2.3.4") == nil
NoMethodError: undefined method `to_u32' for nil:NilClass
        from /usr/local/lib/ruby/gems/2.3/gems/ipaddress-0.8.3/lib/ipaddress/ipv4.rb:512:in `<=>'
        from (irb):2:in `=='
        from (irb):2
        from /usr/local/bin/irb:11:in `<main>'

[smortex]

Hum, this looks like a side effect of the wrong behavior of <=> (see #76).

Reading this old PR, I wonder if it is not just enougth to have the proper <=> which does not produce exception, and == would work as expected (#77 is related).

[asomers]

@smortex that looks like it would work.

[smortex]

OK, therefore I close this PR, and the discussion continues in the related PRs.

Thanks!