Introduce GC'able cache for Roo::Excelx::Extractor#doc (roo-rb#456)

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)
```

lib/roo/excelx/cell/base.rb

@@ -1,12 +1,12 @@
 # frozen_string_literal: true
 
-require "roo/attr_reader_helper"
+require "roo/helpers/default_attr_reader"
 
 module Roo
   class Excelx
     class Cell
       class Base
-        extend Roo::AttrReaderHelper
+        extend Roo::Helpers::DefaultAttrReader
         attr_reader :cell_type, :cell_value, :value
 
         # FIXME: I think style should be deprecated. Having a style attribute

lib/roo/excelx/extractor.rb

@@ -1,8 +1,11 @@
 # frozen_string_literal: true
 
+require "roo/helpers/weak_instance_cache"
+
 module Roo
   class Excelx
     class Extractor
+      include Roo::Helpers::WeakInstanceCache
 
       COMMON_STRINGS = {
         t: "t",
@@ -21,9 +24,11 @@ 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?

lib/roo/helpers/default_attr_reader.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Roo
+  module Helpers
+    module DefaultAttrReader
+      def attr_reader_with_default(attr_hash)
+        attr_hash.each do |attr_name, default_value|
+          instance_variable = :"@#{attr_name}"
+          define_method attr_name do
+            if instance_variable_defined? instance_variable
+              instance_variable_get instance_variable
+            else
+              default_value
+            end
+          end
+        end
+      end
+    end
+  end
+end

lib/roo/helpers/weak_instance_cache.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require "weakref"
+
+module Roo
+  module Helpers
+    module WeakInstanceCache
+      private
+
+      def instance_cache(key)
+        object = nil
+
+        if (ref = instance_variable_get(key)) && ref.weakref_alive?
+          begin
+            object = ref.__getobj__
+          rescue => e
+            unless (defined?(::WeakRef::RefError) && e.is_a?(::WeakRef::RefError)) || (defined?(RefError) && e.is_a?(RefError))
+              raise e
+            end
+          end
+        end
+
+        unless object
+          object = yield
+          instance_variable_set(key, WeakRef.new(object))
+        end
+
+        object
+      end
+    end
+  end
+end