Implement String structure and update Location/Token to use it

v0/token.h

@@ -41,11 +41,7 @@ typedef struct {
 	TokenType token;
 
 	/// @brief The textual representation of a token.
-	/// Note that this is not necessarily null-terminated.
-	char* text;
-
-	/// @brief The length of the `text` string.
-	size_t text_length;
+	String text;
 
 	/// @brief The location of the token.
 	Location location;