Allow converting an ALL_ALLOCATIONS capture file to AGGREGATED_ALLOCATIONS

[godlygeek]

Allow converting a capture file containing information about every allocation to one that contains only information about high water mark and leaked allocations. This could drastically shrink the file, at the cost of throwing away lots of (seldom used) information. (It's also quite useful for convincing yourself that the ALL_ALLOCATIONS format is correct 😅)

https://github.com/godlygeek/memray/tree/transform_to_aggregated contains a working implementation of this, but it's a tough sell architecturally, as it duplicates some code and couples the reader to the writer.

We'll need to decide whether we want this feature enough to justify cleaning it up and implementing it in an architecturally justifiable way.