Rename the GstMemoryImpl to GstMemoryAllocator because that's really what it is.
Add an alloc vmethod to the allocator members.
Improve registration of allocators.
Add methods to get and set the default allocator
Always use an allocator to allocate memory, use the default allocator when NULL
is passed.
Add user_data to the allocator Info so that we can pass extra info to the
allocator new method.
Allow for automatic merging of memory block in the _map function and automatic
freeing of the temporary memory.
Remove some unneeded functions.
Add possibility to force writable spanned memory.