Also make the functions constexpr.
Due to slight changes in floating-point arithmetics (90.0 instead of
180.0, M_PI2 instead of M_PI resp.), results might differ in last
digits (usually 17th decimal digit). This has lead to need to tweak
char2dump's PieChartTest unit test.
Reviewed-by: Tamás Zolnai <email@example.com>
Reviewed-by: Mike Kaganski <firstname.lastname@example.org>