BUG: Compare struct arrays with different endian.

Fixes two places where dtypes with fields are compared for *exact*
equality where they should be compared for *equivalency*.
Merge pull request #3664 from mdboom/structured_array_compare

structured arrays with different byteorders do not compare
ENH: Replace exponentiation with cumulative product in np.vander

Speeds calculation up by ~3x for 100x100 matrices, and by ~45x for
1000x1000
Merge pull request #4568 from jaimefrio/cumprod-vander.

ENH: Replace exponentiation with cumulative product in np.vander