optimize Expr.equal, cache Expr.simplify

[zapashcanon]

Thanks to hashconsing, we have the properties that e1 = e2 <=> e1 == e2, thus we can directly use phys_equal instead of checking the tag in Expr.equal.

I added a cache mechanism to Expr.simplify. As equal and hash are O(1), this is basically free.

The goal is for me to be able to call simplify a lot in Owi (even in cases where it may already have been simplified) without being afraid of re-doing too much work. It's a good way to fix the issue I'm having until we encode the simplification status in the type of expressions.

I believe we could internally optimize this further by sharing a cache between simplify and simplify_expr, but it not as trivial as it seem so I left it out for now.

I quickly checked on Owi and did not find any performance regression with this (and also no improvement but that's because I'm not going simplify that much for now, and because I probably don't have a lot of expressions appearing many times).

[filipeom]

Thanks! 😃