Make str const and add str_slice

src/include/str.h

@@ -3,8 +3,16 @@
 #include <stddef.h>
 
 struct str {
-    char* data; 
-    size_t len;
+    const char*  data; 
+    const size_t len;
 };
 
 #define str_attach(cstr) (struct str){.data = cstr, .len = sizeof(cstr)-1}
+
+static inline struct str str_slice(struct str s, size_t begin, size_t end)
+{
+    return (struct str) {
+        .data = s.data + begin,
+        .len = s.len - begin - end,
+    };
+}