Replace min(), max(), constrain(), round(), etc. macros by functions

[sofian]

min, max, abs, constrain, round and sq are all defined as macros in Arduino which is shitty for a number of reasons. Consider undefining them (#undef) and redefining them as functions.

This is a bit tricky because it might not always be backwards-compatible with Arduino code so we must be very careful in how to implement it.