Use string_view instead of string in Token

cclox_src/token.hh

@@ -3,6 +3,7 @@
 #include "object.hh"
 #include <cstdint>
 #include <string>
+#include <string_view>
 
 enum class TokenType : uint8_t {
   // Single-character tokens.
@@ -31,7 +32,7 @@ struct Token {
   {};
 
   TokenType type;
-  std::string lexeme;
+  std::string_view lexeme;
   Object literal;
   int line;
 };