Add complex number support to linalg.trace

[kgryte]

This PR

• adds support for complex numbers to linalg.trace and follows the guidance set forth in Add complex number support to sum #538.
• specifies that special cases be derived as if the operation was implemented as a reduction over add.
• 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.

[rgommers]

This one is pretty straightforward, and looks consistent with sum. In it goes, thanks @kgryte