[ci skip] Add compatibility functions documentation + Sphinx build fixes for Cart

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+#######################
+Compatibility Functions
+#######################
+
+CodeIgniter provides a set of compatibility functions that enable
+you to use functions what are otherwise natively available in PHP,
+but only in higher versions or depending on a certain extension.
+
+Being custom implementations, these functions will also have some
+set of dependancies on their own, but are still useful if your
+PHP setup doesn't offer them natively.
+
+.. note:: Much like the `common functions <common_functions>`, the
+	compatibility functions are always available, as long as
+	their dependancies are met.
+
+.. contents::
+  :local:
+
+.. raw:: html
+
+  <div class="custom-index container"></div>
+
+****************
+Password Hashing
+****************
+
+This set of compatibility functions offers a "backport" of PHP's
+standard `Password Hashing extension <http://php.net/password>`_
+that is otherwise available only since PHP 5.5.
+
+Dependancies
+============
+
+- PHP 5.3.7
+- ``CRYPT_BLOWFISH`` support for ``crypt()``
+
+Constants
+=========
+
+- ``PASSWORD_BCRYPT``
+- ``PASSWORD_DEFAULT``
+
+Function reference
+==================
+
+.. function:: password_get_info($hash)
+
+	:param	string	$hash: Password hash
+	:returns:	Information about the hashed password
+	:rtype:	array
+
+	For more information, please refer to the `PHP manual for
+	password_get_info() <http://php.net/password_get_info>`_.
+
+.. function:: password_hash($password, $algo[, $options = array()])
+
+	:param	string	$password: Plain-text password
+	:param	int	$algo: Hashing algorithm
+	:param	array	$options: Hashing options
+	:returns:	Hashed password or FALSE on failure
+	:rtype:	string
+
+	For more information, please refer to the `PHP manual for
+	password_hash() <http://php.net/password_hash>`_.
+
+	.. note:: Unless you provide your own (and valid) salt, this function
+		has a further dependancy on an available CSPRNG source. Each
+		of the following would satisfy that:
+		- ``mcrypt_create_iv()`` with ``MCRYPT_DEV_URANDOM``
+		- ``openssl_random_pseudo_bytes()``
+		- /dev/arandom
+		- /dev/urandom
+
+.. function:: password_needs_rehash()
+
+	:param	string	$hash: Password hash
+	:param	int	$algo: Hashing algorithm
+	:param	array	$options: Hashing options
+	:returns:	TRUE if the hash should be rehashed to match the given algorithm and options, FALSE otherwise
+	:rtype:	bool
+
+	For more information, please refer to the `PHP manual for
+	password_needs_rehash() <http://php.net/password_needs_rehash>`_.
+
+.. function:: password_verify($password, $hash)
+
+	:param	string	$password: Plain-text password
+	:param	string	$hash: Password hash
+	:returns:	TRUE if the password matches the hash, FALSE if not
+	:rtype:	bool
+
+	For more information, please refer to the `PHP manual for
+	password_verify() <http://php.net/password_verify>`_.
+
+****************
+Multibyte String
+****************
+
+This set of compatibility functions offers limited support for PHP's
+`Multibyte String extension <http://php.net/mbstring>`_. Because of
+the limited alternative solutions, only a few functions are available.
+
+.. note:: When a character set parameter is ommited,
+	``$config['charset']`` will be used.
+
+Dependancies
+============
+
+- `iconv <http://php.net/iconv>`_ extension
+
+.. important:: This dependancy is optional and these functions will
+	always be declared. If iconv is not available, they WILL
+	fall-back to their non-mbstring versions.
+
+.. important:: Where a character set is supplied, it must be
+	supported by iconv and in a format that it recognizes.
+
+.. note:: For you own dependancy check on the actual mbstring
+	extension, use the ``MB_ENABLED`` constant.
+
+Function reference
+==================
+
+.. function:: mb_strlen($str[, $encoding = NULL])
+
+	:param	string	$str: Input string
+	:param	string	$encoding: Character set
+	:returns:	Number of characters in the input string or FALSE on failure
+	:rtype:	string
+
+	For more information, please refer to the `PHP manual for
+	mb_strlen() <http://php.net/mb_strlen>`_.
+
+.. function:: mb_strpos($haystack, $needle[, $offset = 0[, $encoding = NULL]])
+
+	:param	string	$haystack: String to search in
+	:param	string	$needle: Part of string to search for
+	:param	int	$offset: Search offset
+	:param	string	$encoding: Character set
+	:returns:	Numeric character position of where $needle was found or FALSE if not found
+	:rtype:	mixed
+
+	For more information, please refer to the `PHP manual for
+	mb_strpos() <http://php.net/mb_strpos>`_.
+
+.. function:: mb_substr($str, $start[, $length = NULL[, $encoding = NULL]])
+
+	:param	string	$str: Input string
+	:param	int	$start: Position of first character
+	:param	int	$length: Maximum number of characters
+	:param	string	$encoding: Character set
+	:returns:	Portion of $str specified by $start and $length or FALSE on failure
+	:rtype:	string
+
+	For more information, please refer to the `PHP manual for
+	mb_substr() <http://php.net/mb_substr>`_.
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