Add RRuleSet::set_from_string to support loading rules without DTSTART

[sciyoshi]

This PR adds a new method to RRuleSet called set_from_string which is similar to the FromStr implementation but allows passing rules without a DTSTART line. This is useful in particular when working with Google's API, which provides the rrule without the DTSTART:

As it stands today, because the entire parser module is private, you would need to format a DTSTART line and add it to the rrule (only for it to be parsed again).

I pulled out the common logic in from_str to a private set_from_content_lines method to avoid duplication.

[fmeringdal]

The way this will be used is like this?

RRuleSet::new(date_time)
     .set_from_string(google_rrule_string)

[sciyoshi]

@fmeringdal correct. The name set_from_string could maybe be improved but that's how we plan to use it.

[fmeringdal]

Can you also add an entry to the changelog for this new function and a line about the motivating use-case for it?

[fmeringdal]

@fmeringdal correct. The name set_from_string could maybe be improved but that's how we plan to use it.

It is good enough, naming is hard :)

[sciyoshi]

Can you also add an entry to the changelog for this new function and a line about the motivating use-case for it?

Done 👍

[fmeringdal]

Thanks again. I am aiming for a release later this week that will include these changes.