High memory usage with optimizations

[marvinborner]

As discussed in #1321. With --optimize, Effekt uses a lot of memory. Several times now this led to a Java heap OOM error for me.

Here's a stack trace of one such exception, that disappears with --no-optimize:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
	at scala.collection.Iterator$ConcatIterator.concat(Iterator.scala:1198)
	at scala.collection.Iterator.concat(Iterator.scala:625)
	at scala.collection.Iterator.concat$(Iterator.scala:625)
	at scala.collection.AbstractIterator.concat(Iterator.scala:1306)
	at scala.collection.View$Concat.iterator(View.scala:318)
	at scala.collection.mutable.Growable.addAll(Growable.scala:60)
	at scala.collection.mutable.Growable.addAll$(Growable.scala:57)
	at scala.collection.immutable.SetBuilderImpl.addAll(Set.scala:405)
	at scala.collection.immutable.Set$.from(Set.scala:362)
	at scala.collection.immutable.Set$.from(Set.scala:93)
	at scala.collection.IterableFactoryDefaults.fromSpecific(Iterable.scala:945)
	at scala.collection.IterableFactoryDefaults.fromSpecific$(Iterable.scala:945)
	at scala.collection.AbstractIterable.fromSpecific(Iterable.scala:935)
	at scala.collection.AbstractIterable.fromSpecific(Iterable.scala:935)
	at scala.collection.SetOps.concat(Set.scala:235)
	at scala.collection.SetOps.concat$(Set.scala:226)
	at scala.collection.AbstractSet.concat(Set.scala:269)
	at scala.collection.SetOps.$plus$plus(Set.scala:246)
	at scala.collection.SetOps.$plus$plus$(Set.scala:246)
	at scala.collection.AbstractSet.$plus$plus(Set.scala:269)
	at effekt.core.Type$.$anonfun$22(Type.scala:644)
	at effekt.core.Type$$$Lambda/0x00007f216c310e18.apply(Unknown Source)
	at scala.collection.immutable.List.foldRight(List.scala:353)
	at effekt.core.Type$.typecheckFunctionLike(Type.scala:641)
	at effekt.core.Type$.typecheck$$anonfun$8(Type.scala:602)
	at effekt.core.Type$$$Lambda/0x00007f216c310220.apply(Unknown Source)
	at effekt.core.Type$package$.checking(Type.scala:59)
	at effekt.core.Type$.typecheck(Type.scala:499)
	at effekt.core.Stmt.typing$lzyINIT3(Tree.scala:306)
	at effekt.core.Stmt.typing(Tree.scala:306)
	at effekt.core.Type$.typecheck$$anonfun$5(Type.scala:481)
	at effekt.core.Type$$$Lambda/0x00007f216c3132e0.apply(Unknown Source)

[jiribenes]

Yeah, this is the mostly the core typechecking [on the optimised code [?]] being inefficient :(