fe: library feature current, use thisType of selfType

see also Current:
```
  public Current(SourcePosition pos, AbstractFeature f)
  {
    super(f.thisType());
    ...
  }
```

I found this bug while trying to persist the main application to fum-file then reload it from there.

src/dev/flang/fe/LibraryFeature.java

@@ -615,7 +615,7 @@ AbstractBlock code(int at, Stack<Expr> s, int pos, int posEnd)
             }
           case Current:
             {
-              x = new AbstractCurrent(selfType())
+              x = new AbstractCurrent(selfType().asThis())
                 { public SourcePosition pos() { return LibraryFeature.this.pos(fpos, fposEnd); } };
               break;
             }

src/dev/flang/fe/LibraryModule.java

@@ -101,7 +101,7 @@ public class LibraryModule extends Module implements MirModule
    * The base index of this module. When converting local indices to global
    * indices, the _globalBase will be added.
    */
-  final int _globalBase;
+  private final int _globalBase;
 
 
   /**
@@ -113,19 +113,19 @@ public class LibraryModule extends Module implements MirModule
   /**
    * The module intermediate representation for this module.
    */
-  MIR _mir;
+  private MIR _mir;
 
 
   /**
    * Map from offset in _data to LibraryFeatures for features in this module.
    */
-  TreeMap<Integer, LibraryFeature> _libraryFeatures = new TreeMap<>();
+  private final TreeMap<Integer, LibraryFeature> _libraryFeatures = new TreeMap<>();
 
 
   /**
    * Map from offset in _data to LibraryType for types in this module.
    */
-  TreeMap<Integer, LibraryType> _libraryTypes = new TreeMap<>();
+  private final TreeMap<Integer, LibraryType> _libraryTypes = new TreeMap<>();
 
 
   /**
@@ -141,7 +141,7 @@ public class LibraryModule extends Module implements MirModule
   /**
    * Cache for innerFeatures created from given index
    */
-  Map<Integer, List<AbstractFeature>> _innerFeatures = new TreeMap<>();
+  private final Map<Integer, List<AbstractFeature>> _innerFeatures = new TreeMap<>();
 
 
   /**
@@ -153,13 +153,13 @@ public class LibraryModule extends Module implements MirModule
   /**
    * The universe
    */
-  final AbstractFeature _universe;
+  private final AbstractFeature _universe;
 
 
   /**
    * Modules referenced from this module
    */
-  final ModuleRef[] _modules;
+  private final ModuleRef[] _modules;
 
 
   /**
@@ -375,6 +375,9 @@ AbstractFeature libraryFeature(int offset)
    */
   AbstractFeature feature(int offset)
   {
+    if (PRECONDITIONS) require
+      (offset >= -1);
+
     return
       (offset == -1) ? null :
       (offset ==  0) ? universe()

src/dev/flang/fe/Module.java

@@ -52,7 +52,7 @@
 
 /**
  * A Module represents a Fuzion module independently of whether this is loaded
- * from source code, library from a .mir file or downloaded from the web.
+ * from source code, library from a .fum file or downloaded from the web.
  *
  * @author Fridtjof Siebert ([email protected])
  */
@@ -159,7 +159,7 @@ protected static void add(SortedMap<FeatureName, List<AbstractFeature>> s, Featu
 
 
   /**
-   * Create SourceModule for given options and sourceDirs.
+   * Create Module for given options and sourceDirs.
    */
   Module(LibraryModule[] dependsOn)
   {