Define Schema as a newtype around serde_json::Value

[GREsau]

Implements #242

Still to do (not necessarily all as part of this PR):

• Fix JsonSchema implementations for types in third-party crates
• Remove now-unnecessary preserve_order feature flag
• Where we currently use a single-valued enum, instead use const (check which schema versions support const - may require a new visitor to change const to enum)
• Fix deriving JsonSchema on large types, ensure compile times are OK
• Reduce usages of json!/json_schema! for better performance, particularly in schemars_derive
• Review doc comments
• Update doctests (causing CI to fail)