Change func signatures to match protobuf-c funcs.

Essentially - accept ProtobufCAllocator * args in all exported
functions.  Not yet being used.

Also using ProtobufCMessageDescriptor instead of ProtobufCMessage
when parsing text protobufs so that the resulting ProtobufCMessage
can be freed with protobuf_c_message_free_unpacked (or the relevant
function for the message type.