Function parse_internal 

fn parse_internal<T, D: NetdocParseable>(
    input: &ParseInput<'_>,
    parse_completely: impl FnOnce(&mut ItemStream<'_>) -> Result<T, ErrorProblem>,
) -> Result<T, ParseError>

Common code for parse_netdoc and parse_netdoc_multiple

Creates the ItemStream, calls parse_completely, and handles errors.