Extract interface: TypeParameterDescriptor

A lot of code relied on these descriptors being mutable. In most cases I managed to work around this,
 but there're still are casts to raw List necessary in one or two places

idea/src/org/jetbrains/jet/plugin/JetDescriptorIconProvider.java

@@ -110,7 +110,7 @@ public final class JetDescriptorIconProvider {
             return ((PropertyDescriptor)descriptor).isVar() ? JetIcons.FIELD_VAR : JetIcons.FIELD_VAL;
         }
 
-        if (descriptor instanceof TypeParameterDescriptor) {
+        if (descriptor instanceof TypeParameterDescriptorImpl) {
             return PlatformIcons.CLASS_ICON;
         }
 

idea/src/org/jetbrains/jet/plugin/completion/JetCompletionContributor.java

@@ -183,7 +183,7 @@ public class JetCompletionContributor extends CompletionContributor {
                 DeclarationDescriptor descriptor = ((JetLookupObject)object).getDescriptor();
                 return (descriptor instanceof ClassDescriptor) ||
                        (descriptor instanceof NamespaceDescriptor) ||
-                       (descriptor instanceof TypeParameterDescriptor);
+                       (descriptor instanceof TypeParameterDescriptorImpl);
             }
         }