Support clojure-based migrations

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Closes #162
Closes #164

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Up for debate:

1. Where to put the new code. I think it fits where I put it but you might have other ideas! Feedback welcome.

2. Not supporting mixed types for :up :down, ex:

   {:id "010-bad"
    :up some-ns/some-sym
    :down ["DROP TABLE bar"]}

3. No extra user-friendly error checks.
   The above won't work but we don't explicitly check and present a
   user-friendly error.

4. Failing at migration application time if clj migration fn does not resolve.
   Could try to resolve after load and fail earlier.

5. In the issue there was the idea of adding an option to enable clj migrations (off by default).
   Still want/need?

6. I don't think there is any special work needed for transactions?
   The clojure up down code can decide to use transactions or not.

[weavejester]

Thanks for the PR! To answer your questions:

1. Where you put the code is fine.
2. Mixed types should be supported. Rather than using a separate CljMigration record, you could just put the symbol resolution in SqlMigration.
3. This problem goes away if we support symbols in both :up and :down keys.
4. Failing early sounds like a good idea. The symbol could be resolved on load and passed to SqlMigration as a resolved function.
5. I think we can leave this out for now. We can always put it in later by adding a verification step to the load.
6. I think I agree with this; the function can use a transaction or not.

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Thanks for the review!

2- That was my original inclination, so great!
4- Will do

I'll also address the lint warning I introduced.

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Ok @weavejester, I think I am ready for another review.
Happy to tweak if you notice anything new.

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When you find some time @weavejester, I'd be happy to hear what you think.

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Hi @weavejester! I am still happy to help bring this one to completion, if you find some time/interest.

[weavejester]

Apologies for the delay, this looks okay. Can you ensure any lines you added are 80 characters or less in length, and then squash your commits down. Then it should be good to merge.

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Thanks, @weavejester. I've addressed the 80-character line length feedback for changed lines.

I will squash commits for you after I see CI passing for this commit.

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@weavejester, if you approve this workflow, we can witness it passing, then I can squash my commits as requested.

[weavejester]

Ah, sorry, the workflow button wasn't showing up earlier, so I assumed it had already run.

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No problem, @weavejester. Commits are now squashed.

[weavejester]

Thanks for the PR and apologies for the delays in getting it merged.

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Hi @weavejester, what do you think about cutting a release?

[weavejester]

0.12.0 released. Apologies for the delay.

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Thanks @weavejester!