Introduce K2MetadataCompiler, extract MetadataSerializer out of BuiltInsSerializer

K2MetadataCompiler is a compiler facade similar to K2JVMCompiler and
K2JSCompiler and it produces .kotlin_metadata files. Each .kotlin_metadata file
contains the binary data which is a serialized BuiltIns protobuf message.

There's no 'kotlinc-***' script yet though, so to run this compiler currently
invoke this:

    KOTLIN_COMPILER=org.jetbrains.kotlin.cli.metadata.K2MetadataCompiler kotlinc

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2MetadataCompilerArguments.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2010-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.cli.common.arguments;
+
+import com.sampullara.cli.Argument;
+
+public class K2MetadataCompilerArguments extends CommonCompilerArguments {
+    @Argument(value = "d", description = "Destination for generated .kotlin_metadata files")
+    @ValueDescription("<directory|jar>")
+    public String destination;
+
+    @Argument(value = "classpath", alias = "cp", description = "Paths where to find library .kotlin_metadata files")
+    @ValueDescription("<path>")
+    public String classpath;
+}