context collectors: revision-dependent facts split history #97

Flagged by @MaanasArora in #71 (review) :

I have one main comment, which is that bx compare may not run as-is on the NumPy example. The issue is that revision-dependent facts like numpy_version would split history.
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environment.system splits history (by default), so any version change would split history in the NumPy example. In that case, it doesn't seem like revisions could actually run? (The hash also differs, not just summary.)

I did say collector facts should split history, but it seems I wasn't thinking about run verification yet! I think we should at least change this to allow putting some facts in provenance, the ones which shouldn't be coordinates. We could rename summary to environment and only put that in environment.facts?

There was some back-and-forth in dogfooding numpy collectors in follow-ups to #71, precisely around adding the numpy version. That said, I think it's best to have an initial implementation of main UC-{01,02,03,04} requirements, dogfood them, and then address this issue.