bc: store library in ASCII, shrink keyword parsing code

bc: store library in ASCII, shrink keyword parsing code

Also, use ARRAY_SIZE() where appropriate

function                                             old     new   delta
bc_lib                                              1878    1877      -1
bc_program_index                                      66      64      -2
bc_lex_token                                        1282    1266     -16
bc_lex_kws                                           220     160     -60
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-79)             Total: -79 bytes
   text	   data	    bss	    dec	    hex	filename
 987518	    485	   7296	 995299	  f2fe3	busybox_old
 987440	    485	   7296	 995221	  f2f95	busybox_unstripped

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