Creates a new Parsable subclass (of the given parsable_type) from
the given xml.
An object of the given parsable_type is created, and its <function>pre_parse_xml</function>, <function>
parse_xml</function> and <function>post_parse_xml</function> class functions called on the XML tree obtained from
xml. <function>pre_parse_xml</function> and <function>post_parse_xml</function> are called once each on
the root node of the tree, while <function>parse_xml</function> is called for each of the child nodes of the root node.
If length is -1, xml will be assumed to be null-terminated.
If an error occurs during parsing, a suitable error from ParserError will be returned.
| parsable_type |
the type of the class represented by the XML |
| xml |
the XML for just the parsable object, with full namespace declarations |
| length |
the length of |