move fixed-size determination mostly into libdisasm

*** 43,52 ****
--- 43,53 ----
          dis_disassemble;
          dis_errno;
          dis_handle_create;
          dis_handle_destroy;
          dis_max_instrlen;
+         dis_min_instrlen;
          dis_previnstr;
          dis_set_data;
          dis_flags_set;
          dis_flags_clear;
          dis_strerror;