turned common `tags_as` into static method

this makes it easier to be reused (e.g. by external tools)

note that this can still be used as an instance method:

    SerializationInterface.tags_as(tags) == SerializationInterface().tags_as(tags)

my use case is here:
https://github.com/FND/tiddlywebplugins.tagdex/blob/1302876ac6781dfb7832134707e00b70607d95e3/test/test_web.py#L147
arguably I'm DICW there, so feel free to reject