Use a custom generic tagged union Either instead of std::variant for efficiency (#1476)

* Implement custom generic tagged union `Either`

This should be more efficient than `std::variant`, while still
keeping runtime safety as it `assert`s when `get`ting values.

* Use `Either` for RPN expressions

* Use `Either` for file stack node data

* Use `Either` for `File` buffer

* Use `Either` for `STRFMT` args

* Use `Either` for RGBLINK symbol values

* Support an equivalent of `std::monostate` for `Either`

* Use `Either` for lexer tokens

* Use `Either` for symbol values

* Use `Either` for lexer mmap/buffer state

include/link/symbol.hpp

@@ -7,8 +7,8 @@
 
 #include <stdint.h>
 #include <string>
-#include <variant>
 
+#include "either.hpp"
 #include "linkdefs.hpp"
 
 struct FileStackNode;
@@ -28,14 +28,14 @@ struct Symbol {
 	char const *objFileName;
 	FileStackNode const *src;
 	int32_t lineNo;
-	std::variant<
+	Either<
 	    int32_t, // Constants just have a numeric value
 	    Label    // Label values refer to an offset within a specific section
 	    >
 	    data;
 
-	Label &label();
-	Label const &label() const;
+	Label &label() { return data.get<Label>(); }
+	Label const &label() const { return data.get<Label>(); }
 };
 
 void sym_ForEach(void (*callback)(Symbol &));