Add FromStr impl for Yarn

[ch3pjw]

This is intended to be akin to the FromStr implementation for String found in std::string.

[ch3pjw]

(Just updated because of merge conflicts)

[mcy]

Hmm. I've been mulling this order. I am not a fan of how this not reflect the existing From impl, which is from &'static str since it does not require a copy... do you have a use-case in mind? The only thing it enables is str.parse::<Yarn>() which feels... not super useful?

[ch3pjw]

Basically it enables some generic code that I'm working on that uses FromStr in its interface.

I have a generic interface to build a path structure that parses path segments which can be various types. The (simplified) interface looks like this:

impl<Seg> Path<Seg> {
    fn from_strs<S: AsRef<str>>(
        segs: impl IntoIterator<Item = S>,
    ) -> Result<Self, <Seg as FromStr>::Err>
    where
        Seg: FromStr
    {
        ...
    }
}

Originally I was using Strings for the segment type for a simple Path, but I wanted to use Yarn because it was more efficient for smaller string sizes, which are common in my paths. Yarn was pretty much a drop in replacement for String everywhere, but doesn't conform to this interface, so I thought adding the impl might be of use.

I don't think you'd be able to write a version of FromStr that doesn't use Yarn::copy, if my understanding of lifetimes is correct, because you can't access the lifetime of the given &str in the FromStr interface. (That is, I don't think FromStr could ever be a synonym for From<&'a Str>.) That is at least justification for the difference.

An alternative would be to re-write the interface to use a custom trait, which is not that big a deal. The situation just highlighted that Yarn was not so easy to drop in in this case.