Introduce GC'able cache for Roo::Excelx::Extractor#doc

Memory Profiling script:
```
MemoryProfiler.report{Roo::Excelx.new('test/files/Bibelbund.xlsx').tap{|x|(2..x.last_row).each{|i| x.row(i)}}}.pretty_print

```

Benchmark Script:
```
puts Benchmark.measure{ Roo::Excelx.new('test/files/Bibelbund.xlsx').tap{|x|(2..x.last_row).each{|i| x.row(i)}} }
```


Result on Master:
```
Total allocated: 37131810 bytes (517026 objects)
Total retained:  5562913 bytes (103010 objects)

allocated memory by gem
-----------------------------------
  19288066  roo/lib
  11049821  nokogiri-1.8.4
   6792403  rubyzip-1.2.2
      1304  tmpdir
       216  other

retained memory by gem
-----------------------------------
   5560934  roo/lib
       782  rubyzip-1.2.2
       725  nokogiri-1.8.4
       296  tmpdir
       176  other

  0.720000   0.020000   0.740000 (  0.733750)
```

Result after patch:
```
Total allocated: 34561842 bytes (504998 objects)
Total retained:  5563553 bytes (103026 objects)

allocated memory by gem
-----------------------------------
  19254338  roo/lib
   8513101  nokogiri-1.8.4
   6792403  rubyzip-1.2.2
      1304  tmpdir
       320  weakref
       216  other
       160  ref-2.0.0

retained memory by gem
-----------------------------------
   5561094  roo/lib
       782  rubyzip-1.2.2
       725  nokogiri-1.8.4
       320  weakref
       296  tmpdir
       176  other
       160  ref-2.0.0

  0.610000   0.010000   0.620000 (  0.618642)
```


Note:
Ruby does have native implementation of WeakRef. But I've choosed to go with 'ref' gem for this following reason:
	https://github.com/ruby-concurrency/ref#problems-with-weakref

lib/roo/excelx/extractor.rb

@@ -1,9 +1,10 @@
 # frozen_string_literal: true
 
+require "ref"
+
 module Roo
   class Excelx
     class Extractor
-
       COMMON_STRINGS = {
         t: "t",
         r: "r",
@@ -21,14 +22,31 @@ def initialize(path, options = {})
       private
 
       def doc
-        raise FileNotFound, "#{@path} file not found" unless doc_exists?
+        instance_cache(:@doc) do
+          raise FileNotFound, "#{@path} file not found" unless doc_exists?
 
-        ::Roo::Utils.load_xml(@path).remove_namespaces!
+          ::Roo::Utils.load_xml(@path).remove_namespaces!
+        end
       end
 
       def doc_exists?
         @path && File.exist?(@path)
       end
+
+      def instance_cache(key)
+        object = nil
+
+        if ref = instance_variable_get(key)
+          object = ref.object
+        end
+
+        unless object
+          object = yield
+          instance_variable_set(key, Ref::WeakReference.new(object))
+        end
+
+        object
+      end
     end
   end
 end

roo.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ['lib']
 
   spec.add_dependency 'nokogiri', '~> 1'
+  spec.add_dependency 'ref', '~> 2'
   spec.add_dependency 'rubyzip', '>= 1.2.1', '< 2.0.0'
 
   spec.add_development_dependency 'rake', '~> 10.1'