Denote the optional aspect of types when describing them (#159)

Makes the error message when an optional type is overridden with a non-optional:

```
// Old, broken error message
Error: @aws-cdk/aws-codebuild.Project.role: type changed from @aws-cdk/aws-iam.Role (in @aws-cdk/aws-codebuild.ProjectRef) to @aws-cdk/aws-iam.Role

// New, fixed error message
Error: @aws-cdk/aws-codebuild.Project.role: type changed from @aws-cdk/aws-iam.Role? (in @aws-cdk/aws-codebuild.ProjectRef) to @aws-cdk/aws-iam.Role
```

packages/jsii-spec/lib/spec.ts

@@ -569,20 +569,27 @@ export function isClassOrInterfaceType(type: Type): type is (InterfaceType | Cla
 export function describeTypeReference(a?: TypeReference): string {
     if (a === undefined) { return '(none)'; }
 
+    const optionalMarker = a.optional ? '?' : '';
+
     if (isNamedTypeReference(a)) {
-        return a.fqn;
+        return `${a.fqn}${optionalMarker}`;
     }
 
     if (isPrimitiveTypeReference(a)) {
-        return a.primitive;
+        return `${a.primitive}${optionalMarker}`;
     }
 
     if (isCollectionTypeReference(a)) {
-        return `${a.collection.kind}<${describeTypeReference(a.collection.elementtype)}>`;
+        return `${a.collection.kind}<${describeTypeReference(a.collection.elementtype)}>${optionalMarker}`;
     }
     if (isUnionTypeReference(a)) {
-        return a.union.types.map(describeTypeReference).join('|');
+        const unionType = a.union.types.map(describeTypeReference).join(' | ');
+        if (a.optional) {
+            return `(${unionType})${optionalMarker}`;
+        } else {
+            return unionType;
+        }
     }
 
     throw new Error('Unrecognized type reference');
-}
+}

packages/jsii/tsconfig.json

@@ -44,8 +44,8 @@
     /* Source Map Options */
     // "sourceRoot": "./",                    /* Specify the location where debugger should locate TypeScript files instead of source locations. */
     // "mapRoot": "./",                       /* Specify the location where debugger should locate map files instead of generated locations. */
-    // "inlineSourceMap": false,                  /* Emit a single file with source maps instead of having a separate file. */
-    // "inlineSources": false,                    /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */
+    "inlineSourceMap": true,                  /* Emit a single file with source maps instead of having a separate file. */
+    "inlineSources": true,                    /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */
 
     /* Experimental Options */
     "experimentalDecorators": true            /* Enables experimental support for ES7 decorators. */