Revert "Derive operator!= from operator== (#1660)"

This reverts commit 993879a2ed.

include/gfx/rgba.hpp

@@ -40,6 +40,7 @@ struct Rgba {
 	}
 
 	bool operator==(Rgba const &rhs) const { return toCSS() == rhs.toCSS(); }
+	bool operator!=(Rgba const &rhs) const { return !operator==(rhs); }
 
 	// CGB colors are RGB555, so we use bit 15 to signify that the color is transparent instead
 	// Since the rest of the bits don't matter then, we return 0x8000 exactly.

include/itertools.hpp

@@ -97,6 +97,7 @@ class EnumSeq {
 		EnumT operator*() const { return _value; }
 
 		bool operator==(Iterator const &rhs) const { return _value == rhs._value; }
+		bool operator!=(Iterator const &rhs) const { return !operator==(rhs); }
 	};
 
 public:
@@ -131,6 +132,7 @@ public:
 	bool operator==(ZipIterator const &rhs) const {
 		return std::get<0>(_iters) == std::get<0>(rhs._iters);
 	}
+	bool operator!=(ZipIterator const &rhs) const { return !operator==(rhs); }
 };
 
 // Only needed inside `zip` below.

include/link/section.hpp

@@ -70,6 +70,7 @@ private:
 			SectionT &operator*() const { return *_piece; }
 
 			bool operator==(Iterator const &rhs) const { return _piece == rhs._piece; }
+			bool operator!=(Iterator const &rhs) const { return !operator==(rhs); }
 		};
 
 	public: