Introduce ExhaustiveRefasterTypeMigration check (#770)

The new `@TypeMigration` annotation can be placed on Refaster rule
collections to indicate that they migrate most or all public methods of
an indicated type. The new check validates the claim made by the
annotation.

refaster-support/src/main/java/tech/picnic/errorprone/refaster/annotation/TypeMigration.java

@@ -0,0 +1,37 @@
+package tech.picnic.errorprone.refaster.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Indicates that a Refaster rule or group of Refaster rules is intended to migrate away from the
+ * indicated type.
+ */
+// XXX: Add support for `#unmigratedFields()`.
+// XXX: Consider making this annotation `@Repeatable`, for cases where a single Refaster rule
+// collection migrates away from multiple types.
+@Retention(RetentionPolicy.SOURCE)
+@Target(ElementType.TYPE)
+public @interface TypeMigration {
+  /**
+   * The type migrated away from.
+   *
+   * @return The type generally used in the {@link
+   *     com.google.errorprone.refaster.annotation.BeforeTemplate} methods of annotated Refaster
+   *     rule(s).
+   */
+  Class<?> of();
+
+  /**
+   * The signatures of public methods and constructors that are not (yet) migrated by the annotated
+   * Refaster rule(s).
+   *
+   * @return A possibly empty enumeration of method and constructor signatures, formatted according
+   *     to {@link
+   *     com.google.errorprone.util.Signatures#prettyMethodSignature(com.sun.tools.javac.code.Symbol.ClassSymbol,
+   *     com.sun.tools.javac.code.Symbol.MethodSymbol)}.
+   */
+  String[] unmigratedMethods() default {};
+}