add atan2 to std

[sciconf]

It would be great to add atan2 to std. atan2 is super useful for converting from rectangular to polar coordinates.

It can be written in jsonnet itself, but, it's so simple and ubiquitous that it just seems better to add it with all the other trig functions in std. Thanks!

[sparkprime]

This can go in std.jsonnet presumably?