purgeSlices requires a readonly Context only

Author: mbouaziz

skiplang/prelude/src/skstore/EagerDir.sk

@@ -1832,7 +1832,7 @@ class EagerDir protected {
   }
 
   private fun purgeSlices(
-    context: mutable Context,
+    context: readonly Context,
   ): RangeMapList<Key, DirName> {
     slices = RangeMapList[];
     childDirs = SortedSet[];