Separation of concerns: Buffered file IO with OCaml

This piece of code adds write buffering for append operations on a file, and read buffering to attempt to make repeated preads quicker. Ultimately the implementation supports (buffered) pread, pwrite and (buffered) append.

I'm posting it here because I thought it nicely demonstrated separation of concerns: first we build a file which maintains its own length in memory (V2); then we add the write buffer (V3); then finally we add the read buffer (V4). Each implementation builds on the previous. But each level is reasonably self-contained and addresses the single feature at that level. Previously I tried to do this by implementing everything in a single level, but the code was considerably more complex and harder to follow. 


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