ChaCha20: move working state from ctx to stack

No need to keep it around.
diff --git a/include/mbedtls/chacha20.h b/include/mbedtls/chacha20.h
index 56ee57a..d7a0750 100644
--- a/include/mbedtls/chacha20.h
+++ b/include/mbedtls/chacha20.h
@@ -54,8 +54,7 @@
 
 typedef struct
 {
-    uint32_t initial_state[16];  /*! The initial state (before round operations). */
-    uint32_t working_state[16];  /*! The working state (after round operations). */
+    uint32_t state[16];          /*! The state (before round operations). */
     uint8_t  keystream8[64];     /*! Leftover keystream bytes. */
     size_t keystream_bytes_used; /*! Number of keystream bytes already used. */
 }