Change subject / string type arguments

[joepio]

Many common functions in atomic_lib, like Rersource::new, take a String as an argument.

This has some problems:

• We need a lot of .into_string
• Any functions relating to Subjects are hard to discover (e.g. check if URLs are valid)

We could improve this by:

• Accepting impl From<String>, which deals with &str as well as String. This is the easy approach that already brings some decent DX improvements.
• Accepting a new type for Subject, which implements From<String> and display.

This second Subject type deserves some thoughts:

• Subjects are currently practically always http URLs. Therefore, it might make sense to simply treat them as URLs
• We'll probably introduce a type of local-id in the future. There are (probably) still URLs!
• We could add some useful features, like a method to check if it's a local or not, or what the root of the origin is or something.
• If creating a Subject requires parsing the string, we should consider performance impact, e.g. when constructing a Resource from a binary blob. Same goes for serialisation! So perhaps URLs are a bit too expensive?
• If we use one type for all these subjects, refactoring will be far easier (e.g. if we'll rename subject to id)
• In a recent PR, we already have an AtomicURL type. This creates a URL under the hood, and provides some methods for getting specific routes / subdomains. Perhaps this could be the ID type?