user supplied allocator/printing aka removing stdlib/stdio dependencies

[nopeslide]

currently we are using malloc & printf all over the place,
ignoring platform specific implementation.
I would like to replace these calls so users can provide their own allocators and logging functions

[alrevuelta]

Note that protobuf-c.c also relies on malloc. protobuf_c__allocator can be used to change the alloc and free functions. Can be a good idea to copy the way protobuf-c.c handles allocation.

[nopeslide]

maybe we should provide something like a platform.{c,h} in which we collect all the stdlib/stdio functions we use