Precise dependencies compatibility

[a-alveyblanc]

• Adds HappensAfter data structure
• Adds routine to compute lexicographic order of statements (found in loopy/kernel/dependency.py)
• Updates InstructionBase to accommodate precise dependency semantics (depends_on -> happens_after)
• Other changes to remedy the fact that depends_on is now happens_after

Needs:

• Broaden argument types for UniqueNameGenerator pytools#255 and a pytools release

[a-alveyblanc]

@inducer This is ready for another look.

[inducer]

FWIW, I will fix this up in the interest of getting it in soon.

[inducer]

This snippet here would imply that the order of variables in a set is significant. This is not the case anywhere else in loopy, and I would prefer to not start having it be the case now. This behavior is also not documented.

[inducer]

So... I tried, but I ran out of time. 🙂 There's a bit of algorithmic work to be done here, and some important algorithmic pieces are in #690. We can discuss in more detail during our meeting Monday.

Rather than hijack this PR for my big pile of unrelated changes, that's now #864, which this depends on. The relevant pieces from #690 are also in #864, so you'll be able to rely on them.