Add Node#hash and Node#eql? for correct Hash key behavior

[takahashim]

This PR adds Nokolexbor::Node#hash and Node#eql? for use as Hash keys and Set members.

Ruby requires that a == b implies a.eql?(b) and a.hash == b.hash.
Nokolexbor's Node#== already does C-pointer comparison, but hash and eql? fall back to per-Ruby-object identity.
As a result, two wrappers of the same C node behave inconsistently:

n1 = doc.at_css("div")
n2 = doc.at_css("div")

n1 == n2          # => true
n1.eql?(n2)       # => false
n1.hash == n2.hash # => false

{ n1 => "x" }[n2] # => nil  (lookup miss)

After this PR, eql? matches ==, and hash is derived from the C node pointer, so Hash and Set work as expected.

Example

require "nokolexbor"

doc = Nokolexbor::HTML("<div id='x'><p>hi</p></div>")

n1 = doc.at_css("#x")
n2 = doc.at_css("#x")

cache = { n1 => "value" }
cache[n2]                  # => "value"

[n1, n2, doc.at_css("p")].uniq.size # => 2