Move contexts from SyntaxSet into individual SyntaxReferences

[Enselic]

This is in preparation of lazy-loading syntaxes on-demand. I think this is the most complicated change of the bunch.

Overview of the syntax lazy-loading work:

• Make ContextId implement Hash and simplify the code #377
• Introduce find_unlinked_contexts_in_context() helper #378
• Rename contexts in SyntaxReference to context_ids #379
• Move contexts from SyntaxSet into individual SyntaxReferences #382
• Lazy-load syntax Contexts #393
• Stop compressing the "toplevel" SyntaxSet when serializing it

[Enselic]

The performance impact of this change is ~5% slower deserialization of SyntaxSet. Measured with cargo bench load_internal_dump. This will of course be compensated for several times over in upcoming PRs that introduces lazy-loading.

before:

load_internal_dump      time:   [34.414 ms 34.446 ms 34.485 ms]                              
                        change: [-0.4719% -0.2492% -0.0508%] (p = 0.02 < 0.05)
                        Change within noise threshold.

after:

load_internal_dump      time:   [36.373 ms 36.406 ms 36.451 ms]                              
                        change: [+5.5424% +5.6902% +5.8790%] (p = 0.00 < 0.05)
                        Performance has regressed.

[Enselic]

As you can see, all syntect regression tests pass.
Here is proof all bat regression tests pass with this change: Enselic/bat#51

So risk of behavioral regressions from this PR is low. I would actually go as far as to say "very low".

[trishume]

Looks good! Just one thing which isn't a big deal.

[trishume]

Also I've added you as a collaborator to the repo to recognize you for your valuable contributions. Your PRs have consistently been high-quality, well-explained and well-tested. I also added you to the major contributors listed in the Readme (540555e). Thanks so much, sorry I've been so lazy about reviewing!

What does this mean:

• If you want to review other people's PRs, I'll trust your judgement and just quickly skim them, which will also make me more likely to merge promptly.
• I'd still prefer to review PRs before merging them, but if I'm ever MIA for at least a month you may review PRs and merge them if you like. You may need to ping me for crates.io deploys or to get crates.io access if this happens though.

[Enselic]

Also I've added you as a collaborator to the repo to recognize you for your valuable contributions. Your PRs have consistently been high-quality, well-explained and well-tested. I also added you to the major contributors listed in the Readme (540555e). Thanks so much, sorry I've been so lazy about reviewing!

Thank you! I really appreciate that. On many levels. Don't worry too much about it sometimes taking some days before you get around to doing reviews. It really isn't that bad. I am grateful for that you still are nurturing this important and great project.

A commit to fix your only comment has been pushed.

[Enselic]

@trishume Hi! Just wanted to let you know we are closing in on this being in review for a month. In light of #382 (comment) I will go ahead and merge it after a month has passed.

Let me know if you want more time, that would be absolutely no problem at all :) And of course feel to ask me about anything related to this (or any other) PR!

[trishume]

Thanks for the ping and sorry for the delay. It's been a busy time in my life including some traveling recently. I may have time to do some review Sunday but if I don't then I trust you to do a good job and am happy with you just merging it.