NOTE: This change breaks backwards-compatibility by default.
If you have old callback functions, you can define PB_OLD_CALLBACK_STYLE
to retain the old behaviour.
If you want to convert your old callbacks to new signature, you need
to do the following:
1) Change decode callback argument to void **arg
and encode callback argument to void * const *arg.
2) Change any reference to arg into *arg.
The rationale for making the new behaviour the default is that it
simplifies the common case of "allocate some memory in decode callback".
Update issue 69
Status: FixedInGit
214b0eae8a