math: add a pure V math.mathutil, with generic min, max and abs functions (#9176), and use it consistently

vlib/math/mathutil/mathutil_test.v

@@ -0,0 +1,22 @@
+import math.mathutil as mu
+
+fn test_min() {
+	assert mu.min(42, 13) == 13
+	assert mu.min(5, -10) == -10
+	assert mu.min(7.1, 7.3) == 7.1
+	assert mu.min(u32(32), u32(17)) == 17
+}
+
+fn test_max() {
+	assert mu.max(42, 13) == 42
+	assert mu.max(5, -10) == 5
+	assert mu.max(7.1, 7.3) == 7.3
+	assert mu.max(u32(60), u32(17)) == 60
+}
+
+fn test_abs() {
+	assert mu.abs(99) == 99
+	assert mu.abs(-10) == 10
+	assert mu.abs(1.2345) == 1.2345
+	assert mu.abs(-5.5) == 5.5
+}