+The @PersistentCompilerState@ persists across successive calls to the
+compiler.
+
+ * A ModIface for each non-home-package module
+
+ * An accumulated TypeEnv from all the modules in imported packages
+
+ * An accumulated InstEnv from all the modules in imported packages
+ The point is that we don't want to keep recreating it whenever
+ we compile a new module. The InstEnv component of pcPST is empty.
+ (This means we might "see" instances that we shouldn't "really" see;
+ but the Haskell Report is vague on what is meant to be visible,
+ so we just take the easy road here.)
+
+ * Ditto for rules
+
+ * The persistent renamer state
+