Implement `FromStr` for types

[TehPers]

I think it would be useful if, in addition to (or in place of?) the parse_str methods on the data types (Date, DateTime, Duration, Time), there was a FromStr implementation for the types. The implementation can be simple and just call the equivalent existing parse_str methods:

impl FromStr for Duration {
    type Err = ParseError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        Self::parse_str(s)
    }
}

For Date/DateTime, one question is whether to call parse_str or parse_str_rfc3339. I think it would make sense to call the existing parse_str methods instead of parse_str_rfc3339 since it encompasses the RFC 3339 formats as well, which would be a more permissive FromStr implementation. A user could opt out of this support by calling parse_str_rfc3339 instead.

---

My use case is using these structs in combination with serde (through serde_with). If they implement both Display and FromStr, they can be serialized/deserialized using DisplayFromStr:

#[serde_as]
#[derive(Serialize, Deserialize)]
struct Thing {
    #[serde_as(as = "DisplayFromStr")]
    pub published: Duration,
}

[samuelcolvin]

Sounds good, pr welcome.