feat: add benchmark target to Makefile and optimize isspam.c with stricmp

- Add 'benchmark' target to Makefile that extracts books archive and runs bench.py
- Reorder includes in isspam.c (rstr.h before rstring_list.h)
- Replace file_exists() with case-insensitive stricmp() for forbidden word matching
- Remove unused flags (show_capitalized, show_sentences, etc.) and stripws() function
- Add memory usage comment and fix trailing newline in rstr.h header guard

Makefile

@@ -1,7 +1,7 @@
 CC = gcc 
 CFLAGS = -Wall -Werror -Wextra -Ofast -std=c2x
 
-all: build run valgrind build_risspam run_risspam
+all: build run valgrind build_risspam run_risspam benchmark
 
 build:
 	@echo "Compiling retoor_c project.".
@@ -39,3 +39,10 @@ run_not_spam_risspam:
 
 valgrind: build 
 	valgrind ./isspam ./spam/*.txt
+
+benchmark:
+	-@rm -rf books 
+	echo "Extracting books."
+	tar -xzf books.tar.gz books/
+	echo "Extracted books."
+	python bench.py