implement fmt::Display for SchemaChange

Summary:
## Context

During the investigation of S380145, it was useful to check whether the compiler build time was regressing due to a full build being triggered.

We'd like to log some data on whether a build was incremental or not and if not, why. If we see that we are often falling back to a full builds due to schema changes, we can revisit the classification of schema changes that necessitate a full build.

## This diff

As titled - to log the unsafe schema change that requires a full build

Reviewed By: tyao1

Differential Revision: D51515403

fbshipit-source-id: 9bdc1d8070fd3b0aff6b930a8fd07c61a1826af8

Author: monicatang

compiler/crates/schema-diff/src/definitions.rs

@@ -171,6 +171,17 @@ pub enum SchemaChange {
     DefinitionChanges(Vec<DefinitionChange>),
 }
 
+impl fmt::Display for SchemaChange {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            SchemaChange::None => write!(f, "None"),
+            SchemaChange::GenericChange => write!(f, "GenericChange"),
+            SchemaChange::InvalidSchema => write!(f, "InvalidSchema"),
+            SchemaChange::DefinitionChanges(changes) => write!(f, "{:?}", changes),
+        }
+    }
+}
+
 impl fmt::Debug for SchemaChange {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {