Remove bool return type restriction on Avram::Database#transaction

[matthewmcgarvey]

Fixes #416

This allows for code to use transactions like:

AppDatabase.transaction do
  StepOneOperation.run!
  StepTwoOperation.run!
end

This would only have worked previously if StepTwoOperation.run! returned a boolean. Otherwise true would have to be added as the last line because we required the block to return a Bool.

I believe the original type restriction was put in place to match crystal-db's but it was not intention crystal-lang/crystal-db#145

[jwoertink]

Not too sure about this change. In theory, if it's ok, then we should be able to remove this line and still have things work. I pulled down this PR, and got 3 failures on SaveOperations.

[jwoertink]

Now, as a sort of "hack", we can leave the return type as Bool, but then add a true right after this line. The wrap_in_transaction method will already return the bool for that branch. This lets people put whatever in the block, and we still have that return type. The reason I say "hack" though is, i'm not a huge fan of just have a random static value just chillin there, but It would be a step in the right direction, and no different than what we're doing now 🤔

[BrucePerens]

Wouldn't it be better if this method was declared def transaction : Nil ? Anything that attempted to use the value would get a complaint. Oh, I guess Nil is falsey.

[matthewmcgarvey]

Forgot about this PR but I have updated it to ignore the return value of the yield so that the block isn't forced to return a boolean.