Major overhaul of the Simplifier
This big patch completely overhauls the Simplifier. The simplifier had grown old and crufty, and was hard to understand and maintain. This new version is still quite complicated, because the simplifier does a lot, but it's much easier to understand, for me at least. It does mean that I have touched almost every line of the simplifier, so the diff is a large one. Big changes are these * When simplifying an Expr we generate a simplified Expr plus a bunch of "floats", which are bindings that have floated out of the Expr. Before, this float stuff was returned separately, but not they are embedded in the SimplEnv, which makes the plumbing much easier and more robust. In particular, the SimplEnv already meaintains the "in-scope set", and making that travel with the floats helps to ensure that we always use the right in-scope set. This change has a pervasive effect. * Rather than simplifying the args of a call before trying rules and inlining, we now defer simplifying the args until both rules and inlining have failed, so we're going to leave a call in the result. This avoids the risk of repeatedly simplifying an argument, which was handled by funny ad-hoc flags before. The downside is that we must apply the substitution to the args before rule-matching; and if thep rule doesn't match that is wasted work. But having any rules at all is the exception not the rule, and the substitution is lazy, so we only substitute until a no-match is found. The code is much more elegant though. * A SimplCont is now more zipper-like. It used to have an embedded function, but that was a bit hard to think about, and now it's nice and consistent. The relevant constructors are StrictArg and StrictBind * Each Rule now has an *arity* (gotten by CoreSyn.ruleArity), which tells how many arguments it matches against. This entailed adding a field ru_nargs to a BuiltinRule. And that made me look at PrelRules; I did quite a bit of refactoring in the end, so the diff in PrelRules looks much biggger than it really is. * A little refactoring in OccurAnal. The key change is that in the RHS of x = y `cast` co we regard 'y' as "many", so that it doesn't get inlined into the RHS of x. This allows x to be inlined elsewhere. It's very like the existing situation for x = Just y where we treat 'y' as "many".