Edits to snapshot size argument, minor edits here and there

Following Boud's point in the previous commit, I tried to clarify the point
in the text that we are only talking about hand-written source files: in
short, in this part of the paper, we are not talking abou the
version/snapshot for arXiv which needs figures and many extra automatically
built files. We are just talking about the raw, hand-written files. Trying
to convince people how good it is to keep the raw files separate from
automatically generated files ;-).

Also, while looking around in other parts of the main body of the paper, I
tried to edit/clarify a few points and summarize/shorten others.

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