hush: fix heredoc expansion of $var and `cmd`

hush: fix heredoc expansion of $var and `cmd`

function                                             old     new   delta
expand_pseudo_dquoted                                  -     104    +104
setup_heredoc                                        215     275     +60
done_word                                            669     691     +22
parse_stream                                        1899    1902      +3
setup_redirects                                      196     191      -5
free_pipe                                            189     183      -6
expand_variables                                    2349    2229    -120
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(add/remove: 1/0 grow/shrink: 3/3 up/down: 189/-131)           Total: 58 bytes