Make GstAllocator a GstObject instead of a GstMiniObject, like bufferpool.
Make a new gstallocator.c file. Make a GstAllocator subclass for the default
allocator.
Add a method that memory implementations can call to initialize the standard
GstMemory structure.
Move the parent handling in the _free handler.
Rearrange some internal function parameters so that the order is consistent.
Add more memory examples
