Conditions with field number described but not supported

[DifferentialOrange]

#109 introduced a detailed guide on selects.

crud/doc/select.md

Line 5 in fbe4de0

``CRUD`` allows to filter tuples by conditions. Each condition can use field name (or number) or index name. The first condition that uses index name is used to iterate over space. If there is no conditions that match index names, full scan is performed. Other conditions are used as additional filters. Search condition for the indexed field must be placed first to avoid a full scan.

describes field number as a valid conditions operand, but it is not.

crud/crud/compare/conditions.lua

Lines 121 to 126 in fbe4de0

	local operand = user_condition[2]
	if type(operand) ~= 'string' then
	return nil, ParseConditionError:new(
	"condition[2] should be string, got %q (condition %s)", type(operand), i
	)
	end

permits to use only string operands.

Adding a test case with field number also causes test run to fail

--- a/test/unit/parse_conditions_test.lua
+++ b/test/unit/parse_conditions_test.lua
@@ -12,6 +12,7 @@ g.test_parse = function()
         {'<=', 'dd', {1, 2, 3}},
         {'>', 'eeeeee', 666},
         {'>=', 'f', {3, 3, 4}},
+        {'==', 1, nil},
     }
 
     local conditions, err = compare_conditions.parse(user_conditions)
@@ -23,6 +24,7 @@ g.test_parse = function()
         cond_funcs.le('dd', {1, 2, 3}),
         cond_funcs.gt('eeeeee', 666),
         cond_funcs.ge('f', {3, 3, 4}),
+        cond_funcs.eq(1, nil),
     })
 end

./.rocks/bin/luatest ./test/unit/parse_conditions_test.lua 
"tuple.keydef" module is used
"tuple.merger" module is used
F..

Failed tests:
-------------

1) parse_conditions.test_parse
...hub/tarantool/crud///test/unit/parse_conditions_test.lua:19: expected: nil, actual: ParseConditionError: condition[2] should be string, got "number" (condition 7)
stack traceback:
	...opment/github/tarantool/crud/crud/compare/conditions.lua:123: in function 'parse'
	...hub/tarantool/crud///test/unit/parse_conditions_test.lua:18: in function 'method'
	...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:339: in function <...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:338>
	[C]: in function 'xpcall'
	...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:338: in function 'super'
	...antool/crud/.rocks/share/tarantool/luatest/capturing.lua:106: in function 'protected_call'
	...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:400: in function 'super'
	.../tarantool/crud/.rocks/share/tarantool/luatest/hooks.lua:175: in function 'invoke_test_function'
	...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:395: in function 'super'
	...antool/crud/.rocks/share/tarantool/luatest/capturing.lua:124: in function 'run_test'
	...
	[C]: in function 'xpcall'
	.../tarantool/crud/.rocks/share/tarantool/luatest/utils.lua:37: in function 'run_tests'
	...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:257: in function <...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:253>
	[C]: in function 'xpcall'
	...antool/crud/.rocks/share/tarantool/luatest/capturing.lua:74: in function <...antool/crud/.rocks/share/tarantool/luatest/capturing.lua:72>
	[C]: in function 'xpcall'
	...tarantool/crud/.rocks/share/tarantool/luatest/runner.lua:49: in function 'fn'
	...crud/.rocks/share/tarantool/luatest/sandboxed_runner.lua:14: in function 'run'
	...l/crud/.rocks/share/tarantool/luatest/cli_entrypoint.lua:4: in function <...l/crud/.rocks/share/tarantool/luatest/cli_entrypoint.lua:3>
	....rocks/share/tarantool/rocks/luatest/0.5.5-1/bin/luatest:3: in main chunk