Reorganize the field decoder interface.

This makes the field decoding functions more intuitive to use.
The old interface is still present if you specify NANOPB_INTERNALS.

Update issue 2
Status: FixedInGit

tests/decode_unittests.c

@@ -1,3 +1,5 @@
+#define NANOPB_INTERNALS
+
 #include <stdio.h>
 #include <string.h>
 #include "pb_decode.h"