Optimize the HStore parser

Test string from taken from the the test suite: `"\"a\\\\b\"=>\"b\\\\ar\", \"1\\\"foo\"=>\"2\""`

Bench:

```
Warming up --------------------------------------
            original     6.896k i/100ms
             patched    15.787k i/100ms
Calculating -------------------------------------
            original     68.176k (± 1.2%) i/s -    344.800k in   5.058270s
             patched    157.786k (± 1.0%) i/s -    789.350k in   5.003144s

Comparison:
             patched:   157786.0 i/s
            original:    68176.1 i/s - 2.31x  (± 0.00) slower

```

Memory before: allocated 4376 bytes (55 objects)
Memory after: allocated: 1880 bytes (23 objects)

activerecord/lib/active_record/connection_adapters/postgresql/oid/hstore.rb

@@ -13,11 +13,17 @@ def type
 
           def deserialize(value)
             if value.is_a?(::String)
-              ::Hash[value.scan(HstorePair).map { |k, v|
-                v = v.upcase == "NULL" ? nil : v.gsub(/\A"(.*)"\Z/m, '\1').gsub(/\\(.)/, '\1')
-                k = k.gsub(/\A"(.*)"\Z/m, '\1').gsub(/\\(.)/, '\1')
-                [k, v]
-              }]
+              hash = {}
+              value.scan(HstorePair) do |key, value|
+                key.gsub!('\"', '"')
+                key.gsub!('\\\\', '\\')
+
+                value&.gsub!('\"', '"')
+                value&.gsub!('\\\\', '\\')
+
+                hash[key] = value
+              end
+              hash
             else
               value
             end
@@ -47,9 +53,8 @@ def changed_in_place?(raw_old_value, new_value)
 
           private
             HstorePair = begin
-              quoted_string = /"[^"\\]*(?:\\.[^"\\]*)*"/
-              unquoted_string = /(?:\\.|[^\s,])[^\s=,\\]*(?:\\.[^\s=,\\]*|=[^,>])*/
-              /(#{quoted_string}|#{unquoted_string})\s*=>\s*(#{quoted_string}|#{unquoted_string})/
+              quoted_string = /"([^"\\]*(?:\\.[^"\\]*)*)"/
+              /#{quoted_string}\s*=>\s*(?:(?=NULL)|#{quoted_string})/
             end
 
             def escape_hstore(value)

activerecord/test/cases/adapters/postgresql/hstore_test.rb

@@ -111,8 +111,8 @@ def test_cast_value_on_write
   def test_type_cast_hstore
     assert_equal({ "1" => "2" }, @type.deserialize("\"1\"=>\"2\""))
     assert_equal({}, @type.deserialize(""))
-    assert_equal({ "key" => nil }, @type.deserialize("key => NULL"))
-    assert_equal({ "c" => "}", '"a"' => 'b "a b' }, @type.deserialize(%q(c=>"}", "\"a\""=>"b \"a b")))
+    assert_cycle("key" => nil)
+    assert_cycle("c" => "}", '"a"' => 'b "a b')
   end
 
   def test_with_store_accessors
@@ -198,48 +198,36 @@ def test_hstore_dirty_from_database_equal
     assert_not_predicate hstore, :changed?
   end
 
-  def test_gen1
-    assert_equal('" "=>""', @type.serialize(" " => ""))
+  def test_spaces
+    assert_cycle(" " => " ")
   end
 
-  def test_gen2
-    assert_equal('","=>""', @type.serialize("," => ""))
+  def test_commas
+    assert_cycle("," => "")
   end
 
-  def test_gen3
-    assert_equal('"="=>""', @type.serialize("=" => ""))
+  def test_signs
+    assert_cycle("=" => ">")
   end
 
-  def test_gen4
-    assert_equal('">"=>""', @type.serialize(">" => ""))
+  def test_various_null
+    assert_cycle({ "a" => nil, "b" => nil, "c" => "NuLl", "null" => "c" })
   end
 
-  def test_parse1
-    assert_equal({ "a" => nil, "b" => nil, "c" => "NuLl", "null" => "c" }, @type.deserialize('a=>null,b=>NuLl,c=>"NuLl",null=>c'))
-  end
-
-  def test_parse2
-    assert_equal({ " " => " " },  @type.deserialize("\\ =>\\ "))
-  end
-
-  def test_parse3
-    assert_equal({ "=" => ">" },  @type.deserialize("==>>"))
-  end
-
-  def test_parse4
-    assert_equal({ "=a" => "q=w" },   @type.deserialize('\=a=>q=w'))
+  def test_equal_signs
+    assert_cycle("=a" => "q=w")
   end
 
   def test_parse5
-    assert_equal({ "=a" => "q=w" },   @type.deserialize('"=a"=>q\=w'))
+    assert_cycle("=a" => "q=w")
   end
 
   def test_parse6
-    assert_equal({ "\"a" => "q>w" },  @type.deserialize('"\"a"=>q>w'))
+    assert_cycle("\"a" => "q>w")
   end
 
   def test_parse7
-    assert_equal({ "\"a" => "q\"w" }, @type.deserialize('\"a=>q"w'))
+    assert_cycle("\"a" => "q\"w")
   end
 
   def test_rewrite