Remove Descriptor from MinidumpMemoryBase/UnifiedMemory

[Swatinem]

Working on #944 , I also found an annoyance / inefficiency with UnifiedMemory:

UnifiedMemory is an enum containing MinidumpMemoryBase with different generic parameters, so we are doing a match on that as well on each access.

MINIDUMP_MEMORY_DESCRIPTOR/64 are both 16 bytes, so we might as well just wrap the Descriptor in an enum, and remove all the generic parameters from MinidumpMemory(List)Base.

This should clean up the code with less generics, and also avoid a match in the hot code path. I doubt any hot code path ever cares about the inner descriptor, so we might as well remove it completely from MinidumpMemoryBase, and move it someplace else.

Another observation is that MinidumpMemoryBase.size is pretty redundant with MinidumpMemoryBase.bytes.len(), so might as well remove that too?