Make term-level rewrite use bottom-up order by default; closes #3

Bottom-up rewriting is safer (less prone to divergence) and often more
natural, so it’s a better default behavior.

Note that this is potentially a breaking change, though I expect that
most code that uses `rewrite` will still work.

Currently some tests are broken; they will be fixed in the next commit.

core/src/main/scala/squid/ir/RuleBasedTransformer.scala

@@ -82,7 +82,7 @@ class RuleBasedTransformerMacros(val c: whitebox.Context) {
 
     val recursive = c.macroApplication.symbol.annotations.exists(_.tree.tpe <:< typeOf[RecRewrite])
     val RuleBasedTrans = if (recursive) tq"_root_.squid.ir.FixPointRuleBasedTransformer" :: tq"_root_.squid.ir.BottomUpTransformer" :: Nil
-      else tq"_root_.squid.ir.SimpleRuleBasedTransformer" :: tq"_root_.squid.ir.TopDownTransformer" :: Nil
+      else tq"_root_.squid.ir.SimpleRuleBasedTransformer" :: tq"_root_.squid.ir.BottomUpTransformer" :: Nil
 
     //val $transName = new _root_.scp.ir.SimpleRuleBasedTransformer with _root_.scp.ir.TopDownTransformer {
     //val $transName: _root_.scp.ir.Transformer{val base: ${base.tpe} } = new _root_.scp.ir.SimpleTransformer {