Audit public APIs

[hawkw]

Before releasing tokio-trace-core 0.1, we should give a thorough pass to all the publicly exposed traits, functions, types, and fields in the core crate.

Some things to consider:

• Does it need to be part of the public API surface?
• Public fields should be replaced with accessor functions whenever possible (so that we can change internal representations without causing a breaking change).
• For types:
  • If it's an enum, is it intended to be matched exhaustively? If not, can it be made private, wrapped in a struct, and replaced with methods to test/construct individual variants (such as the subscriber error types)?
  • Are there traits in std that the type should implement (i.e., error types should implement std::error::Error)?
• For traits:
  • Are there types in std that should implement the trait (i.e. Option<Id> can implement AsId, etc)?
  • Is it intended to be implemented by users of the crate? If not, it should probably be sealed so it cannot be implemented.