C++ midpoint and lerp functions.

This gives us well-behaved interpolation for numbers and pointers

These just got voted into C++20!

float a = 3.2f; float b = 7.8f; float t = 0.75; auto x = std::lerp(a, b, t);

Jonathan Wakely just put this in (3/3/2019)! W00t!

But I can still play around. ;-) Plus, IMHO, we should have these for complex as well.