Implement memory-based caching

[kevmo314]

Right now, we still write to disk instead of committing to memory with mmap. We should explicitly not write to disk until the user requests. This is a bit more complicated but turns out disk is indeed a pretty big bottleneck :(

[kevmo314]

The key idea here is we want to add an interface that accepts an offset and an BinaryMarshaller, and don't write it to disk but instead store it in a map[uint64]any. Then when the file is closed, write those all to disk. For reads, check if it exists in the cache already and if it does, return the untyped struct. If it doesn't, do the BinaryUnmarshalling