src/bigint.c: implement multi-precision integer.

To enable multi-precision integer support, you need to link
`mruby-bigint` mrbgem. The gem itself is empty but it turns on
the "bigint" support.

src/array.c

@@ -847,7 +847,7 @@ ary_subseq(mrb_state *mrb, struct RArray *a, mrb_int beg, mrb_int len)
     return mrb_ary_new_from_values(mrb, len, ARY_PTR(a)+beg);
   }
   ary_make_shared(mrb, a);
-  b  = MRB_OBJ_ALLOC(mrb, MRB_TT_ARRAY, mrb->array_class);
+  b = MRB_OBJ_ALLOC(mrb, MRB_TT_ARRAY, mrb->array_class);
   b->as.heap.ptr = a->as.heap.ptr + beg;
   b->as.heap.len = len;
   b->as.heap.aux.shared = a->as.heap.aux.shared;