bc: use more compact parsing data structures

bc: use more compact parsing data structures

function                                             old     new   delta
dc_lex_token                                         697     701      +4
bc_parse_next_rel                                     20       -     -20
bc_parse_next_read                                    20       -     -20
bc_parse_next_print                                   20       -     -20
bc_parse_next_param                                   20       -     -20
bc_parse_next_for                                     20       -     -20
bc_parse_next_expr                                    20       -     -20
bc_parse_next_elem                                    20       -     -20
common_parse_expr                                     62      40     -22
bc_parse_expr                                         49      24     -25
dc_lex_regs                                           52      13     -39
bc_parse_name                                        581     539     -42
bc_parse_expr_empty_ok                              2157    2108     -49
dc_parse_insts                                       332      83    -249
dc_lex_tokens                                        364      91    -273
bc_parse_stmt                                       2261    1868    -393
------------------------------------------------------------------------------
(add/remove: 0/7 grow/shrink: 1/8 up/down: 4/-1232)         Total: -1228 bytes
   text	   data	    bss	    dec	    hex	filename
 987037	    485	   7296	 994818	  f2e02	busybox_old
 985814	    485	   7296	 993595	  f293b	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>