WIP: refactor ilp tag

Still quite a bit to do for this, but need some high level feedback at this stage, namely:

• I'm replacing "ilp.seq" tag with "like parallel", which pushes loop to innermost. What do we do with "ilp" aka "ilp.unr"?

• I'm exposing the functionality to privatize temporaries, do we expect user to call it explicitly? (If they forgot to do so, they will get a race condition warning), see https://gitlab.tiker.net/inducer/loopy/compare/master...refactor-ilp#a3740609c363b89c60afd5c8730dfcd9b4b5fed0_223_221

• What distinguishes between "like parallel" and "vector" tag?

Many thanks!