Missing asin, acos, atan and one form of pow

[ryankaplan]

Hi @sibaku!

I've been this library in a personal project. Thanks for building it! I've ended up adding stuff to it, like...

• HessNum3 a_asin(in HessNum3 a) which is like asin(float a)
• HessNum3 a_acos(in HessNum3 a) which is like acos(float a)
• HessNum3 a_atan(in HessNum3 a) which is like atan(float a)
• apow(in HessNum3 a, in HessNum3 b)

I'd be happy to open a PR that adds these. But I figured I'd ask first to see if you're open to it!

I've implemented these only for HessNum3. But I would implement them on all data types for consistency.

Let me know what you think!