Improve information available from diffs

This change adds a lot more details to the result of a ModelDiff,
including access to the raw Differences between the models, access to
the old and new models through the Differences object, access to the old
validation events and new validation events, and access to the diff
events. Helper methods were added to ModelDiff.Result to detect which
validation events were introduced by a change,  which validation
events were resolved by a change, and if there are breaking changes in
the diff (e.g., events that are ERROR or DANGER). Note that any kind of
suppression of diff events would need to be applied and managed outside
of this helper functionality.

Added some missing equals and hashCode implementations.

smithy-diff/src/main/java/software/amazon/smithy/diff/ChangedMetadata.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License").
  * You may not use this file except in compliance with the License.
@@ -15,6 +15,7 @@
 
 package software.amazon.smithy.diff;
 
+import java.util.Objects;
 import software.amazon.smithy.model.FromSourceLocation;
 import software.amazon.smithy.model.SourceLocation;
 import software.amazon.smithy.model.node.Node;
@@ -64,4 +65,23 @@ public Node getNewValue() {
     public SourceLocation getSourceLocation() {
         return getNewValue().getSourceLocation();
     }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        } else if (!(o instanceof ChangedMetadata)) {
+            return false;
+        } else {
+            ChangedMetadata that = (ChangedMetadata) o;
+            return getKey().equals(that.getKey())
+                   && Objects.equals(getOldValue(), that.getOldValue())
+                   && Objects.equals(getNewValue(), that.getNewValue());
+        }
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(getKey(), getOldValue(), getNewValue());
+    }
 }

smithy-diff/src/main/java/software/amazon/smithy/diff/ChangedShape.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License").
  * You may not use this file except in compliance with the License.
@@ -18,6 +18,7 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.stream.Stream;
 import software.amazon.smithy.model.FromSourceLocation;
@@ -138,6 +139,26 @@ public Map<ShapeId, Pair<Trait, Trait>> getTraitDifferences() {
         return traitDiff;
     }
 
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        } else if (!(o instanceof ChangedShape)) {
+            return false;
+        } else {
+            // If the shapes are equal, then the changed traits are equal, so
+            // there's no need to compare the traitDiff property.
+            ChangedShape<?> that = (ChangedShape<?>) o;
+            return Objects.equals(getOldShape(), that.getOldShape())
+                   && Objects.equals(getNewShape(), that.getNewShape());
+        }
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(getOldShape(), getNewShape());
+    }
+
     /**
      * Finds the trait differences between the old and new shape.
      *

smithy-diff/src/main/java/software/amazon/smithy/diff/Differences.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License").
  * You may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Objects;
 import java.util.stream.Stream;
 import software.amazon.smithy.model.Model;
 import software.amazon.smithy.model.node.Node;
@@ -161,6 +162,25 @@ public Stream<ChangedMetadata> changedMetadata() {
         return changedMetadata.stream();
     }
 
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        } else if (!(o instanceof Differences)) {
+            return false;
+        } else {
+            // The differences between the models are always equivalent if the
+            // models are equivalent, so no need to compare them.
+            Differences that = (Differences) o;
+            return getOldModel().equals(that.getOldModel()) && getNewModel().equals(that.getNewModel());
+        }
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(getOldModel(), getNewModel());
+    }
+
     private static void detectShapeChanges(Model oldModel, Model newModel, Differences differences) {
         for (Shape oldShape : oldModel.toSet()) {
             newModel.getShape(oldShape.getId()).ifPresent(newShape -> {