Adds new methods to index_object_parts (#5140)

* Since 3.1.0 4 new ```key_def``` methods are applicable to ```index_object.parts```
   * ```validate_key()```
   * ```validate_full_key```
   * ```validate_tuple()```
   * ```compare_keys()```
* Functionally methods are equivalent to their description in ```key_def```
* Fixes #5139

* Fixes syntax

Author: AArdeev

doc/reference/reference_lua/box_index/parts.rst

@@ -15,25 +15,31 @@ index_object.parts
     :ref:`compare_with_key() <key_def-compare_with_key>`,
     :ref:`merge() <key_def-merge>`.
 
+    Since version :doc:`3.1.0 </release/3.1.0>`, the ``index_object.parts`` can operate methods
+    :ref:`validate_key() <key_validate_key>`,
+    :ref:`validate_full_key() <key_validate_full_key>`,
+    :ref:`validate_tuple() <key_validate_tuple>`,
+    :ref:`compare_keys() <key_compare_keys>`.
+
     **``index_object.parts`` example**
 
     ..  code-block:: lua
 
-            box.schema.space.create('T')
-            i = box.space.T:create_index('I', {parts={3, 'string', 1, 'unsigned'}})
-            box.space.T:insert{1, 99.5, 'X', nil, 99.5}
-            i.parts:extract_key(box.space.T:get({'X', 1}))
+        box.schema.space.create('T')
+        i = box.space.T:create_index('I', {parts={3, 'string', 1, 'unsigned'}})
+        box.space.T:insert{1, 99.5, 'X', nil, 99.5}
+        i.parts:extract_key(box.space.T:get({'X', 1}))
 
     **``key_def`` equivalent**
 
-        ..  code-block:: lua
+    ..  code-block:: lua
 
-            key_def = require('key_def')
-            box.schema.space.create('T')
-            i = box.space.T:create_index('I', {parts={3, 'string', 1, 'unsigned'}})
-            box.space.T:insert{1, 99.5, 'X', nil, 99.5}
-            k = key_def.new(i.parts)
-            k:extract_key(box.space.T:get({'X', 1}))
+        key_def = require('key_def')
+        box.schema.space.create('T')
+        i = box.space.T:create_index('I', {parts={3, 'string', 1, 'unsigned'}})
+        box.space.T:insert{1, 99.5, 'X', nil, 99.5}
+        k = key_def.new(i.parts)
+        k:extract_key(box.space.T:get({'X', 1}))
 
     The outcome of the methods calling is described in :ref:`key_def_object <key_def_object>`.