libc: Add strspn, strcspn, and strtok declarations to string.h

libc/include/string.h

@@ -47,6 +47,15 @@ extern "C"
     /* Return the human-readable description of an error number. */
     char* strerror(int errnum);
 
+    /* Return the length of the initial segment of str which consists only of bytes in accept. */
+    usize strspn(const char* str, const char* accept);
+
+    /* Return the length of the initial segment of str which consists only of bytes not in reject. */
+    usize strcspn(const char* str, const char* reject);
+
+    /* Separate a string into several tokens. */
+    char* strtok(char* str, const char* delim);
+
 #ifdef __cplusplus
 }
 #endif