core/macros: add math helper macros

[benpicco]

Contribution description

This adds a few commonly useful math helper macros to core/macros:

• SIGNOF() returns the sign of an integer
• DIV_ROUND() performs arithmetic integer division
• DIV_ROUND_UP() performs arithmetic integer division, always rounding to the next integer larger in magnitude.

Testing procedure

unittests/tests-core was extended with test cases for the new macros

Issues/PRs references

[kaspar030]

... and they are supposed to be used in compile-time situations, I guess?

[benpicco]

Also at run-time.
It's just that I've found myself open-coding those way too often, so I thought providing a macro would be appropriate.

[maribu]

RIOT/cpu/stm32/periph/ptp.c

Lines 63 to 71 in 6db9960

	/**
	* @brief Return the rounded result of x / y
	* @param x Number to divide
	* @param y @p x should be divided by this
	* @return x/y, rounded to the nearest integer, using "round half up" rule
	* @pre Both @p x and @p y are compile time constant integers and the
	* expressions are evaluated without side-effects
	*/
	#define ROUNDED_DIV(x, y) (((x) + ((y) / 2)) / (y))

It's just that I've found myself open-coding those way too often, so I thought providing a macro would be appropriate.

Yes, indeed. Is just one of the cases where this would have been useful.