Implement `powi` and `powf` for `ArrayBase`

[multimeric]

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Currently a ** 3 is the only example in the "ndarray_for_numpy_users" docs that requires a map. Could these methods be added directly? Rust has no ^ or ** exponentiation operator, but these two methods are implemented on numeric primitives, and I see no reason why they can't be added for arrays here for convenience.