Add parsed_as? method

This PR provides a standard way to check how a lens parses a string, in order to add logical branching for lens versions, such as we did for the shellvar lens in 20af628b6d7ade0535ac7c9de3e799ebafebc449.

Also, it adds a tempfile-based block when `#text_store` is not available, so all tests pass now, even with ruby-augeas 0.3.0.