Improve performance of include? by 5-10x

Rails uses IPAddr#include? to evaluate what it should use as the
client's remote ip by filtering potential ips against a trusted list
of internal ips. In a _very_ minimal app, #include? was showing up in
a profile as ~1% of request time.

The issue is that #include? was converting itself and the other value
passed in to ranges of IPAddr. This mean as a worst case (where other is
a non-IPAddr, like a String) then there would be 5 IPAddr instances
created (other -> IPAddr, and two each for the conversions to ranges).
However, wrapping the begin and end values as IPAddr is not needed
because they are necessarily fixed addresses already.

This patch extracts the logic for getting the begin_addr and end_addr
from the #to_range method so that they can be used in #include? without
having to instantiate so many IPAddr.

Benchmark:

```ruby
net1 = IPAddr.new("192.168.2.0/24")
net2 = IPAddr.new("192.168.2.100")
net3 = IPAddr.new("192.168.3.0")
net4 = IPAddr.new("192.168.2.0/16")

Benchmark.ips do |x|
  x.report("/24 includes address") { net1.include? net2 }
  x.report("/24 not includes address") { net1.include? net3 }
  x.report("/16 includes /24") { net4.include? net1 }
  x.report("/24 not includes /16") { net1.include? net4 }
  x.compare!
end
```

Before:

```
Comparison:
    /24 not includes /16:   175041.3 i/s
/24 not includes address:   164933.2 i/s - 1.06x  (± 0.00) slower
        /16 includes /24:   163881.9 i/s - 1.07x  (± 0.00) slower
    /24 includes address:   163558.4 i/s - 1.07x  (± 0.00) slower
```

After:

```
Comparison:
    /24 not includes /16:  2588364.9 i/s
/24 not includes address:  1474650.7 i/s - 1.76x  (± 0.00) slower
        /16 includes /24:  1461351.0 i/s - 1.77x  (± 0.00) slower
    /24 includes address:  1425463.5 i/s - 1.82x  (± 0.00) slower
```

lib/ipaddr.rb

@@ -176,9 +176,7 @@ class IPAddr
   def include?(other)
     other = coerce_other(other)
     return false unless other.family == family
-    range = to_range
-    other = other.to_range
-    range.begin <= other.begin && range.end >= other.end
+    begin_addr <= other.begin_addr && end_addr >= other.end_addr
   end
   alias === include?
 
@@ -406,17 +404,6 @@ class IPAddr
 
   # Creates a Range object for the network address.
   def to_range
-    begin_addr = (@addr & @mask_addr)
-
-    case @family
-    when Socket::AF_INET
-      end_addr = (@addr | (IN4MASK ^ @mask_addr))
-    when Socket::AF_INET6
-      end_addr = (@addr | (IN6MASK ^ @mask_addr))
-    else
-      raise AddressFamilyError, "unsupported address family"
-    end
-
     self.class.new(begin_addr, @family)..self.class.new(end_addr, @family)
   end
 
@@ -497,6 +484,21 @@ class IPAddr
 
   protected
 
+  def begin_addr
+    @addr & @mask_addr
+  end
+
+  def end_addr
+    case @family
+    when Socket::AF_INET
+      @addr | (IN4MASK ^ @mask_addr)
+    when Socket::AF_INET6
+      @addr | (IN6MASK ^ @mask_addr)
+    else
+      raise AddressFamilyError, "unsupported address family"
+    end
+  end
+
   # Set +@addr+, the internal stored ip address, to given +addr+. The
   # parameter +addr+ is validated using the first +family+ member,
   # which is +Socket::AF_INET+ or +Socket::AF_INET6+.