Function formatter_field_name_split

unicode_formatter_field_name_split is used to implement string.Formatter.vformat. it takes an PEP 3101 "field name", and returns a tuple of (first, rest): "first", the part before the first '.' or '['; and "rest", an iterator for the rest of the field name. it's a wrapper around stringlib/string_format.h's field_name_split. The iterator it returns is a FieldNameIterator */