hush: speed up o_addX{chr,str}

hush: speed up o_addX{chr,str}

function                                             old     new   delta
o_addQstr                                              -     162    +162
o_addQchr                                              -      89     +89
o_addstr                                               -      58     +58
o_addqchr                                             50      81     +31
expand_on_ifs                                        103      97      -6
add_till_backquote                                    92      82     -10
expand_variables                                    1281    1217     -64
parse_stream                                        1675    1609     -66
o_addqstr                                            155       -    -155
------------------------------------------------------------------------------
(add/remove: 3/1 grow/shrink: 1/4 up/down: 340/-301)           Total: 39 bytes
   text    data     bss     dec     hex filename
 759870     604    6684  767158   bb4b6 busybox_old
 759909     604    6684  767197   bb4dd busybox_unstripped