Conform Quaternion to Elementary Functions

[markuswntr]

This extends https://github.com/apple/swift-numerics/pull/206 and adds the remaining quaternionic elementary functions; making Quaternion fully conform to ElementaryFunctions. 🎉 Most of the functions draw inspiration from the implementation details of the corresponding elementary function on complex numbers. Nonetheless, I greatly appreciate every comment, feedback or suggestions!

Links to new elementary functions:

• log
• log1p
• acos
• asin
• atan
• acosh
• asinh
• atanh
• pow of Quaternion
• pow of Int
• sqrt
• root

[markuswntr]

The implementation is still functional, but I am trying to improve the error bounds on log and maybe add an executable to validate the worst-case error (much like the ComplexLog tests), and so I would like to leave it as a draft for now.