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.
Instead of just checking that the line of the error message is correct,
assert that start and end position are on the correct character offset.
Also fix all the tests to conform to this.
