Resolver implementation use of unsafe code is not sound

[yamadapc]

Resolver has the following unsafe pieces of code:

• Lifetimes casts due to use of Arenas for String interning
• Mutable string cast to remove JSON comments from JSON string

This is not sound & likely causes undefined behaviour on certain cases.

It's likely unnecessary:

• We can parse JSON into an owned structure that isn't borrowing from the input
• We can use a JSON5 parser to handle comments and trailing commas