Eckard's test case:
cat <<! | cat
asdf
!
The issue is that we have to continue parsing until we see
the '\n' before consuming the here document.
So we revert to the old approach of having two functions:
heredoc() which remembers if we'v seen a heredoc redirection
and a second readhere() function that reads the doc from
the lexers input and sets Tree.str on thee REDIR node.
369cba5f93