Proposal: Add complex cis function

[Expander]

I propose to add the complex variant of the real $\mathrm{cis}$ function. It would be defined as $\mathrm{cis}\colon\mathbb{C}\to\mathbb{C}$ with

$$\mathrm{cis}(z) = \exp(iz) = \exp(-\mathrm{Im}(z))\mathrm{cis}(\mathrm{Re}(z))$$

This complex $\mathrm{cis}$ function has applications in physics and numerical mathematics.

[cuviper]

Since there's already fn cis(T) -> Complex<T>, maybe this could be fn into_cis(self) -> Self? Is there any naming precedent from other languages for this form?

[Expander]

Implementations in other languages that I found so far include:

• Intel's C++ compiler has only double _Complex cis(double). (I guess in C's naming convention one would have cis for a function with a real and ccis for a function with a complex argument.)
• Common Lisp has only cis real => complex
• julia has cis(z::Real) and cis(z::Complex) via function overloading
• D has Complex!real expi(real y)

So, it could also be named expi or ccis.