This old implementation, which was meant to be revised but which we
forgot about, caused a surprising slowdown: if x were a traced array of
size 50000, evaluating len(x) would create 50000 traced temporary
objects, which led to a lot of overhead! That came up in our
implementation of jax.random.shuffle, which happened to call len()
instead of x.shape[axis] (even though it should have been using x.size
anyway, according to tjablin@'s code that it's based on).
