Ensure ShapedArray.shape is always a tuple of builtins integers
Currently, it can sometimes include elements of type int64, e.g.,
In [1]: import jax.numpy as jnp
In [2]: x = jnp.arange(3) + 1
In [3]: x.shape # looks fine at first glance
Out[3]: (3,)
In [4]: type(x.shape[0]) # yikes!
Out[4]: numpy.int64
This confirms my hypothesis that NumPy's scalar types are the root of all evil.
