Deserialize from string by default

[baszalmstra]

We often write code like this:

#[serde_as(as = "Vec<DisplayFromStr>")]
pub constrains: Vec<MatchSpec>,

This is because the default serde implementation serializes all the fields. However, I think its more common to serialize matchspec as a string then to serialize the individual fields. I would therefor propose that we serialize and deserialize matchspecs as strings instead.

This is also the case for NamelessMatchSpec and Version and possibly some other types.