- Support negative exponents in Array `matrix_pow(A, n)`.
- Support negative exponents in Array `matrix_pow(A, n)`.

Example:

	say [[3,9],[5,7]]**(-2)		#=> [[47/288, -5/32], [-25/288, 3/32]]

This is equivalent with:

	say inv([[3,9],[5,7]]**2)	#=> [[47/288, -5/32], [-25/288, 3/32]]

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