Move DeclarationDescriptor.substitute to separate interface Substitutable

To get rid of pointless/confusing implementations in ModuleDescriptor,
PackageViewDescriptor, TypeParameterDescriptor and others.

Note that there are still implementations that do not make sense, for
example in those subclasses of VariableDescriptor which are not also
subclasses of CallableMemberDescriptor (e.g. ValueParameterDescriptor).
Those can be removed by making CallableMemberDescriptor (instead of
CallableDescriptor) inherit from Substitutable. However, that would
require more changes in the compiler because CallableDescriptor is used
rather often in places where in fact only CallableMemberDescriptor
instances can appear.

Explicit return types and casts are required in some places now because
there's no single non-trivial supertype for
ClassifierDescriptorWithTypeParameters and CallableDescriptor.
Previously it was DeclarationDescriptorWithVisibility, now it's both
that and Substitutable<...>

compiler/backend/src/org/jetbrains/kotlin/codegen/context/RootContext.java

@@ -23,7 +23,6 @@ import org.jetbrains.kotlin.descriptors.DeclarationDescriptor;
 import org.jetbrains.kotlin.descriptors.DeclarationDescriptorVisitor;
 import org.jetbrains.kotlin.descriptors.annotations.Annotations;
 import org.jetbrains.kotlin.name.Name;
-import org.jetbrains.kotlin.types.TypeSubstitutor;
 
 public class RootContext extends CodegenContext<RootContext.FakeDescriptor> {
     private final GenerationState state;
@@ -56,11 +55,6 @@ public class RootContext extends CodegenContext<RootContext.FakeDescriptor> {
             throw new IllegalStateException();
         }
 
-        @Override
-        public DeclarationDescriptor substitute(@NotNull TypeSubstitutor substitutor) {
-            throw new IllegalStateException();
-        }
-
         @Override
         public <R, D> R accept(DeclarationDescriptorVisitor<R, D> visitor, D data) {
             throw new IllegalStateException();