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/asm/lexer.hpp

@@ -8,9 +8,9 @@
 #include <optional>
 #include <stdint.h>
 #include <string>
-#include <variant>
 #include <vector>
 
+#include "either.hpp"
 #include "platform.hpp" // SSIZE_MAX
 
 // This value is a compromise between `LexerState` allocation performance when `mmap` works, and
@@ -98,7 +98,7 @@ struct LexerState {
 	bool expandStrings;
 	std::deque<Expansion> expansions; // Front is the innermost current expansion
 
-	std::variant<std::monostate, ViewedContent, BufferedContent> content;
+	Either<ViewedContent, BufferedContent> content;
 
 	~LexerState();