Reduce memory usage when writing files (#3526)
This PR reduces the our total memory usage (hopefully) for files that we
write to disk.
When we write a file, we obviously have the input bytes (the bytes we
want to write to disk). But we also read the file to see if the file
needs to be written at all. This means we have _another_ copy of the
bytes, the ones that came from the disk.
Worse, our write is strongly dependent on the read's `RopeVc`. Meaning
we're stuck with that read memory until some indeterminate time. And any
time we update the written contents, we're gonna get more bytes stuck in
our Vc cache.
The goal here is to track the file, so that we can rewrite it if it's
modified/deleted. Why not have an API specific to tracking, without
actually reading any contents? This PR does exactly that. And now that
we don't need to read contents to track, we can avoid using the `read()`
API (and its `RopeVc`) in order to do our new-old contents comparison.
