Add hp_mana_units.hpp helper

Source/utils/hp_mana_units.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+namespace devilution {
+
+constexpr int HpManaFracBits = 6;
+constexpr int HpManaScale = 1 << HpManaFracBits;
+
+constexpr int HpManaToFrac(int whole)
+{
+	return whole * HpManaScale;
+}
+
+constexpr int HpManaToWhole(int frac)
+{
+	return frac / HpManaScale;
+}
+
+constexpr int HpManaFromParts(int whole, int frac)
+{
+	return HpManaToFrac(whole) + frac;
+}
+
+} // namespace devilution