update Counter, and refactor schema creation logic

[heckj]

For "bound" types - Counter and AutomergeText, only the AutomergeText initializer that takes a "path" will create the schema (if it doesn't exist and doesn't conflict). That logic is a bit of mess, and really needs to be extracted, and a similiar mechanism used for Counter - which still requires schema in an Automerge Document to be created before invoking bind() on it.