Remove unmodifiable collection wrappers in ResolveContext

This fixes #700 (Java is unhelpful and the Collections.unmodifiableList etc
methods don't check whether the thing is already wrapped).

These were just an assertion basically intended to find any accidental
modification of these collections, so not essential.

config/src/main/java/com/typesafe/config/impl/ResolveContext.java

@@ -32,8 +32,12 @@ final class ResolveContext {
         this.memos = memos;
         this.options = options;
         this.restrictToChild = restrictToChild;
-        this.resolveStack = Collections.unmodifiableList(resolveStack);
-        this.cycleMarkers = Collections.unmodifiableSet(cycleMarkers);
+        // we don't defensively copy/wrap these because Collections.unmodifiableList etc.
+        // nest infinitely in a way that causes stack overflow (they don't check to avoid
+        // multiple wrappers). But they should be treated as immutable because they end
+        // up shared between multiple ResolveContext.
+        this.resolveStack = resolveStack;
+        this.cycleMarkers = cycleMarkers;
     }
 
     private static Set<AbstractConfigValue> newCycleMarkers() {