Function parse_optional_payload 

pub fn parse_optional_payload<'a>(
    lexer: &mut Lexer<'a>,
    ctor: impl FnOnce(Option<Box<Type<'a>>>, SourceSpan) -> Type<'a>,
) -> ParseResult<Type<'a>>

Consumes the current keyword token, parses an optional <T> payload, then builds the type using it’s constructor.