Make ContextId implement Hash and simplify the code

[Enselic]

I thought it would be a good idea to split up #374 into smaller and self-contained parts that are easier to digest and make sense of. Here is the first such PR. It passes

• syntect regression tests
• bat regression tests

and I know from working with the referenced PR that the code is very sensitive, and the slightest mistake will practically always make at least one test fail. So that all tests pass gives good confidence that the code is in order.

As for the change itself; making ContextId implement Hash allows us to make the code simpler and clearer. Most importantly:

• We don't need to pass around a cryptic HashSet<usize>. We can instead pass
  around the semantically much clearer HashSet<ContextId>,

But I also take the opportunity to:

• Remove the ContextId::index() getter. ContextId shall be seen as a
  primitive that we pass around, rather than something with getters.

• Stop using ContextId::new(). The way we use it, it is just boilerplate.
  (On a related note: Long-term, we should even remove it from the public API. Clients should never
  create their own ContextIds.)

[trishume]

Sorry for not reviewing this for so long, my life has been busy lately. This looks good!