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.iml

@@ -21,5 +21,6 @@
     <orderEntry type="module" module-name="js.serializer" />
     <orderEntry type="module" module-name="util" />
     <orderEntry type="module" module-name="annotation-collector" />
+    <orderEntry type="module" module-name="builtins-serializer" />
   </component>
 </module>