common/: Move common modules to `lazycat/'

* common/message.scm, common/protocol.scm, common/proxy.scm,
  common/translator.scm, common/utils.scm: Move to `lazycat/'.
* lazycat/Makefile.am (pkgdata_DATA): Add common modules.
* configure.ac (AC_CONFIG_FILES): Update.
* Makefile.am (SUBDIRS): Update.
* common/: Remove.