bc: create a few common functions

bc: create a few common functions

function                                             old     new   delta
bc_char_vec_init                                       -      12     +12
bc_vec_pop_all                                         -       8      +8
bc_program_index                                      64      66      +2
bc_vec_string                                         64      61      -3
bc_vec_free                                           27      24      -3
bc_read_line                                         317     314      -3
bc_program_execStr                                   564     561      -3
bc_program_exec                                     4153    4150      -3
bc_program_copyToVar                                 340     336      -4
bc_parse_create                                      175     170      -5
dc_lex_token                                         705     699      -6
bc_program_reset                                      78      72      -6
bc_lex_number                                        305     299      -6
bc_program_read                                      353     343     -10
bc_program_addFunc                                   225     209     -16
bc_vm_run                                           1942    1924     -18
bc_parse_reset                                       181     163     -18
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 1/14 up/down: 22/-104)          Total: -82 bytes
   text	   data	    bss	    dec	    hex	filename
 987600	    485	   7296	 995381	  f3035	busybox_old
 987518	    485	   7296	 995299	  f2fe3	busybox_unstripped

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