Implement the functions described in the "Mathematical
Expressions" section of the "CSS Values and Units Module
Level 4" spec, https://www.w3.org/TR/css-values-4/.
Beyond calc(), which we already had, this includes
min(), max(), clamp(),
round(), rem(), mod(),
sin(), cos(), tan(), asin(), acos(), atan(), atan2(),
pow(), sqrt(), hypot(), log(), exp(),
abs(), sign(),
e, pi, infinity and NaN.
Some tests included.
