Add Tagset::parse_symbol_string for convenience parsing of comma-separated symbol strings

Fast and drity (regex-based) XCES parser based on the one in Pantera, noticeably faster but prone to breakage on even slightly freeform xml files

update AnnotationChannel interface, fix make_iob_from_segments bug, upgrade make_annotation_vector, annotation heads

ignore consecutive colons in tag parsing, so parsing 'subst::::' throws an attribute-missing error instead of a unknown-value