bson: make bson_init_static() return a boolean.
iter: add bson_iter_recurse() to iterate a sub document or array.

This allows you to iterate children documents or arrays on the stack
using another bson_iter_t.

bson_iter_t iter;
bson_iter_t child;

if (bson_iter_init_find(&iter, bson, "key")) {
   if (bson_iter_recurse(&iter, &child)) {
      /* do things with sub document. */
   }
}