More math functions/constants

[rben01]

I believe the following don't yet exist in jsonnet. If not, it would be great if they were added:

• pi = 3.141592653589793
• deg2rad(x) = pi/180 * x
• rad2deg(x) = 180/pi * x
• atan2(y, x)
• pow2, pow10 (it's nice to have unary functions instead of needing to define the lambda)
• log2, log10 (easier and shorter than remembering that log_a(b) = log(b)/log(a))
• round(x, places=0) to customize how many decimals to keep
• hypot(x, y) = sqrt(x*x + y*y)

[johnbartholomew]

Corresponding PR #1187 has been merged.