Increase (matrix, utils).h modularity.

Ref:
	Perform general header cleanup by moving macros and struct definitions that
	are meant to be private from header to source files.

	src/matrix.(c, h)
		-insert forward declaration and typedef of struct Matrix in matrix.h,
		and move definition of Matrix to matrix.c
		-move RAD (degree-radians conversion) macro to matrix.c, as it's only
		used by the matrix functions and no other C module.

	src/utils.(c, h)
		-move ABS() substitution macro from utils.h to utils.c, because it's
		only used by the utils functions.