Add `FromStr` implementation to ease integration with `parse-display`

[fogti]

reference: DieracDelta/flake_generator#8
Currently, we (flake_generator contributors) resorted to wrapping SmolStr in a SmlStr wrapper for implementing FromStr, but I think this should be upstreamed.
e.g.

impl FromStr for SmolStr {
    type Err = std::convert::Infallible;

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