Implement `serde::(De)Serialize` for `keyvalues_parser::Vdf`

[CosmicHorrorDev]

Implementing this would be a huge ergonomics win for users, but beyond that it would unlock much better fuzzing potential for keyvalues-serde. Right now keyvalues-serde currently fuzzes an arbitrarily generated "kitchen sink" style struct that holds a lot of different data types, but that doesn't catch issues like #45 :crying_cat_face:

Having this implemented would give us the arbitrarily dynamic type that would be perfect for fuzzing to find since it's the one type that describes all valid VDF blobs

[CosmicHorrorDev]

We'll also want to implement it for Value to complete things. We should also be able to allow map types for the top-level pair. We just have to keep track of if we're in the top level in the deserializer, so that we don't won't wind up allowing things like

struct Foo {
    bar: Pair,
}

as valid