Create an implementation-independent data format or parser for the Web of Trust

For now the file import/export is a direct (de)serialization of LegacyWebOfTrust. To allow new implementations to be used in place of old ones, this import/export must not depend on implementation details.

This adds 2 functions:

• from_file<T: WebOfTrust>(path: &str, &mut wot: T ) -> Result<(), SomeErrorType>
• to_file<T: WebOfTrust>(path: &str, &wot: T) -> Result<(), SomeOtherErrorType>