Add complex number support to prod

[kgryte]

This PR

• adds support for computing the product of complex numbers. As described in Add complex number support for multiplication #551, for complex numbers having finite components, complex number multiplication is well-defined; however, special cases involving NaN and infinite components may diverge depending on how one chooses to model complex infinity.
• specifies that special cases be derived as if the operation was implemented as a reduction over multiply.
• requires that, if x is a complex floating-point data type and dtype is None, the function must return an array having the default complex floating-point data type.
• updates the input and output array data types to be any numeric data type, not just real-valued data types.
• depends on Add complex number support for multiplication #551.

[rgommers]

The PR this depended on is merged now, and this LGTM - so in it goes. Thanks @kgryte