RFC: add `angle` for computing the argument of a complex number

[steff456]

This RFC requests to include a new API in the array API specification for the purpose of computing the argument of a complex number.

Overview

Based on array comparison data, the API is available in the majority of libraries in the PyData ecosystem.

The Array API specification does not currently include an API for computing the argument of a complex number.

Prior art

• NumPy: https://numpy.org/doc/stable/reference/generated/numpy.angle.html
• PyTorch: https://pytorch.org/docs/stable/generated/torch.angle.html
• TensorFlow: https://www.tensorflow.org/api_docs/python/tf/math/angle
• C99: https://en.cppreference.com/w/c/numeric/complex/carg

Proposal:

def angle(x: array, /) -> array

Questions

• okay with angle as name? arg? phase?
• only operate on complex arrays, similar to conj et al?

cc @kgryte