Various minor improvements

[wilcoxjay]

I have recently developed two small SpaceSearch tools (a Sudoku solver and a automated Hoare logic tool for Imp). This PR collects some library-level improvements to SpaceSearch that I factored out along the way.

Here are the main changes.

• I moved countUp from the Queens example into the SpaceSearch library, since I needed it in another tool as well. I also refactored it and zCountUp to use a generic list function.
• I added infix notation for various operations on Int.
• I added infix notation for Ensemble.In, which I was surprised @konne88 hadn't done yet 😉
• I improved some of the new tactic automation.
• I added a new library function called guard, which lifts a predicate (A -> bool) to a monadic arrow A -> Space A by requiring the predicate to hold (returning empty otherwise). I refactored Queens to use this in one place, and I also use it in both my new examples.

This PR is a bit of a smorgasbord. I'm happy to split it up if people don't like certain parts. Feedback welcome!