Remove raptor_turtle_check_uri_string() which is at wrong level.

Needs to be checked in earlier lexing so e.g. illegal escapes in URIs
such as \b are found.  For example.
(raptor_ntriples_parse_term_internal): Enforce URI restrictions.

URIs may not have \t \b \n \r \f or raw ' ' or \u0020 or \u003C or \u003E
(raptor_stringbuffer_append_turtle_string): Enforce URI restrictions.

Add is_uri argument to distinguish.  Report type label correctly.
URIs may not have \t \b \n \r \f or raw ' ' or \u0020 or \u003C or \u003E

Update all raptor_stringbuffer_append_turtle_string callers to pass URI flag.
Rename raptor_turtle_expand_name_escapes to raptor_turtle_expand_qname_escapes
Add ntriples test bug-577 for URI escaping