Extend built-in Nasal math.

Extend built-in Nasal math.

- rename mod() to fmod() to prevent collisions with the
version in math.nas (which has different handling of negatives)
- implement pow, tan, acos and asin natively
- add round(x, [p]), which rounds away from 0.0, and takes an
option precision. I.e you can round(479, 50) and get '500'; useful
in many digital cockpit displays.