Implement Ord for span ID

## Feature Request

### Crates

`tracing_core`

### Motivation

`Subscriber` implementations might want to store spans in a `BTreeMap`. This requires span IDs to implement `Ord`. Since they're just `u64`s this should be trivial.

### Proposal

Just derive `Ord`.

### Alternatives

Use `.into_u64()` all over the place.