PERF: use arr.size instead of np.prod(arr.shape) in _can_use_numexpr
xref https://github.com/pandas-dev/pandas/pull/39772

Using `np.prod` gives quite some overhead, and I *think* it's always equivalent to `.size` ?

```
In [1]: arr = np.random.randn(3, 2)

In [2]: %timeit arr.size
42.8 ns ± 0.628 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [3]: %timeit np.prod(arr.shape)
6.63 µs ± 344 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
```