hush: fix some TODOs. TODO in export builting: +250 bytes.

hush: fix some TODOs. TODO in export builting: +250 bytes.
 simplify unexpected EOF handling.

function                                             old     new   delta
builtin_export                                       117     374    +257
o_reset_to_empty_unquoted                              -      21     +21
parse_stream                                        1926    1929      +3
syntax_error_unterm_ch                                31      32      +1
handle_dollar                                        644     641      -3
parse_stream_dquoted                                 307     298      -9
add_till_backquote                                   106      97      -9
add_till_closing_paren                               303     286     -17
o_reset                                               21       -     -21
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 3/4 up/down: 282/-59)           Total: 223 bytes