Inherit Roo::Excelx::Coordinate from Array

Profiling Script
```
MemoryProfiler.report{ Roo::Excelx.new(file_name).tap{|x|(2..x.last_row).each{|i| x.row(i)}} }.pretty_print
```

Master:
```
Total allocated: 34559100 bytes (504994 objects)
Total retained:  5563403 bytes (103022 objects)

allocated memory by gem
-----------------------------------
  19254338  roo/lib
   8509100  nokogiri-1.8.4
   6793822  rubyzip-1.2.2
      1304  tmpdir
       320  weakref
       216  other

allocated objects by gem
-----------------------------------
    358381  roo/lib
    145846  nokogiri-1.8.4
       735  rubyzip-1.2.2
        22  tmpdir
         8  weakref
         2  other

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

retained objects by gem
-----------------------------------
    102989  roo/lib
        14  nokogiri-1.8.4
         8  weakref
         7  rubyzip-1.2.2
         3  tmpdir
         1  other
```

After Patch:
```
Total allocated: 33439850 bytes (477013 objects)
Total retained:  4444153 bytes (75041 objects)

allocated memory by gem
-----------------------------------
  18135098  roo/lib
   8509100  nokogiri-1.8.4
   6793812  rubyzip-1.2.2
      1304  tmpdir
       320  weakref
       216  other

allocated objects by gem
-----------------------------------
    330400  roo/lib
    145846  nokogiri-1.8.4
       735  rubyzip-1.2.2
        22  tmpdir
         8  weakref
         2  other

retained memory by gem
-----------------------------------
   4441854  roo/lib
       782  rubyzip-1.2.2
       725  nokogiri-1.8.4
       320  weakref
       296  tmpdir
       176  other

retained objects by gem
-----------------------------------
     75008  roo/lib
        14  nokogiri-1.8.4
         8  weakref
         7  rubyzip-1.2.2
         3  tmpdir
         1  other
```

lib/roo/excelx/coordinate.rb

@@ -1,15 +1,18 @@
 module Roo
   class Excelx
-    class Coordinate
-      attr_accessor :row, :column
+    class Coordinate < ::Array
 
       def initialize(row, column)
-        @row = row
-        @column = column
+        super() << row << column
+        freeze
       end
 
-      def to_a
-        @array ||= [row, column].freeze
+      def row
+        self[0]
+      end
+
+      def column
+        self[1]
       end
     end
   end

lib/roo/excelx/sheet_doc.rb

@@ -44,8 +44,7 @@ def each_cell(row_xml)
           # If you're sure you're not going to need this hyperlinks you can discard it
           coordinate = ::Roo::Utils.extract_coordinate(cell_element[COMMON_STRINGS[:r]])
           hyperlinks = unless @options[:no_hyperlinks]
-                         key = coordinate.to_a
-                         hyperlinks(@relationships)[key]
+                         hyperlinks(@relationships)[coordinate]
                        end
 
           yield cell_from_xml(cell_element, hyperlinks, coordinate)
@@ -200,8 +199,7 @@ def extract_cells(relationships)
         extracted_cells = {}
         doc.xpath('/worksheet/sheetData/row/c').each do |cell_xml|
           coordinate = ::Roo::Utils.extract_coordinate(cell_xml[COMMON_STRINGS[:r]])
-          key = coordinate.to_a
-          extracted_cells[key] = cell_from_xml(cell_xml, hyperlinks(relationships)[key], coordinate)
+          extracted_cells[coordinate] = cell_from_xml(cell_xml, hyperlinks(relationships)[coordinate], coordinate)
         end
 
         expand_merged_ranges(extracted_cells) if @options[:expand_merged_ranges]

lib/roo/utils.rb

@@ -4,7 +4,7 @@ module Utils
 
     LETTERS = ('A'..'Z').to_a
 
-    def extract_coord(s, klass = Excelx::Coordinate)
+    def extract_coordinate(s)
       num = letter_num = 0
       num_only = false
 
@@ -22,20 +22,16 @@ def extract_coord(s, klass = Excelx::Coordinate)
       end
       fail ArgumentError if letter_num == 0 || !num_only
 
-      if klass == Array
-        [num, letter_num]
-      else
-        klass.new(num, letter_num)
-      end
+      Excelx::Coordinate.new(num, letter_num)
     end
 
-    alias_method :extract_coordinate, :extract_coord
+    alias_method :ref_to_key, :extract_coordinate
 
     def split_coordinate(str)
-      extract_coord(str, Array)
+      warn "[DEPRECATION] `Roo::Utils.split_coordinate` is deprecated.  Please use `Roo::Utils.extract_coordinate` instead."
+      extract_coordinate(str)
     end
 
-    alias_method :ref_to_key, :split_coordinate
 
 
     def split_coord(str)
@@ -72,8 +68,8 @@ def num_cells_in_range(str)
       cells = str.split(':')
       return 1 if cells.count == 1
       raise ArgumentError.new("invalid range string: #{str}. Supported range format 'A1:B2'") if cells.count != 2
-      x1, y1 = split_coordinate(cells[0])
-      x2, y2 = split_coordinate(cells[1])
+      x1, y1 = extract_coordinate(cells[0])
+      x2, y2 = extract_coordinate(cells[1])
       (x2 - (x1 - 1)) * (y2 - (y1 - 1))
     end