Do not support GCC 9 (#1978)

This will let us use C++20 features that GCC 9's experimental
C++20 support did not yet cover, such as "concepts".

This reverts some commits:
- 6bcd79b997
- d5ce5329ea
- 728d14879b

include/gfx/rgba.hpp

@@ -40,7 +40,6 @@ 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

@@ -97,7 +97,6 @@ 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:
@@ -132,7 +131,6 @@ 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

@@ -70,7 +70,6 @@ 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:

include/util.hpp

@@ -74,14 +74,14 @@ char const *printChar(int c);
 
 struct Uppercase {
 	// FNV-1a hash of an uppercased string
-	size_t operator()(std::string const &str) const {
+	constexpr size_t operator()(std::string const &str) const {
 		return std::accumulate(RANGE(str), size_t(0x811C9DC5), [](size_t hash, char c) {
 			return (hash ^ toUpper(c)) * 16777619;
 		});
 	}
 
 	// Compare two strings without case-sensitivity (by converting to uppercase)
-	bool operator()(std::string const &str1, std::string const &str2) const {
+	constexpr bool operator()(std::string const &str1, std::string const &str2) const {
 		return std::equal(RANGE(str1), RANGE(str2), [](char c1, char c2) {
 			return toUpper(c1) == toUpper(c2);
 		});