RFC: add `proj` for computing the projection of a complex number

[steff456]

This RFC requests to include a new API in the array API specification for the purpose of computing the projection of a complex number on the Riemann sphere.

Overview

The Array API specification currently leaves the model for complex number infinities unspecified, which can be especially problematic when testing for complex number equality. Adding support for proj can help resolve ambiguity by mapping all complex number infinities to a single infinity (i.e., positive real infinity).

Prior art

• C99: https://en.cppreference.com/w/c/numeric/complex/cproj
• (not currently implemented by array libraries)

Proposal:

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

Questions

• Only operate on complex arrays, similar to conj et al?

cc @kgryte