Implement FromStr for ids

[onlycs]

Useful for databasing, i.e. in Postgres, where ID's must be stored as strings because u64::MAX is larger than the largest BigInt type.

// before
Id::from(some_string.parse::<u64>().unwrap());

// after
Id::from_str(&some_string).unwrap();

[tazz4843]

Unrelated to this PR, but for Postgres, you can simply cast to a i64 and it'll be fine even once the 64th bit is used as long as you cast back as well.

[scottbot95]

you can simply cast to a i64 and it'll be fine even once the 64th bit is used as long as you cast back as well.

And here I was foolishly using std::mem::transmute to "re-interpret" the u64 as an i64 cuz I just assumed the cast would fail or otherwise behave in an undesirable way....