Add compile-time option PB_BUFFER_ONLY.

This allows slight optimizations if only memory buffer support
(as opposed to stream callbacks) is wanted. On ARM difference
is -12% execution time, -4% code size when enabled.

pb_decode.h

@@ -32,7 +32,16 @@ extern "C" {
  */
 struct _pb_istream_t
 {
+#ifdef PB_BUFFER_ONLY
+    /* Callback pointer is not used in buffer-only configuration.
+     * Having an int pointer here allows binary compatibility but
+     * gives an error if someone tries to assign callback function.
+     */
+    int *callback;
+#else
     bool (*callback)(pb_istream_t *stream, uint8_t *buf, size_t count);
+#endif
+
     void *state; /* Free field for use by callback implementation */
     size_t bytes_left;