Rework Entity parsing implementation, foundation for comment improvements

[danbroooks]

This PR refactors the way that the initial entity definition blocks are parsed by the parse function before they then get turned into an UnboundEntityDef. This is an initial change that hopefully will make it easier to implement #1297 and #1296.

I have added an example test file that illustrates various possible comment styles and what the parse currently is or isn't picking up. This test can be used when tackling the issues linked above. The content of this test is here:

https://github.com/yesodweb/persistent/blob/54be9d68125984249587f81971269f1c7c599a4a/persistent/test/Database/Persist/TH/CommentsSpec.hs

I'm not sure exactly what behavior is expected for comments, so I'd welcome any input on what the behavior should be as I might have made some assumptions that are not desired for the library. But I'd assume everything I've included there persistent would want to be parsed as comments.

The only actual behavioral change regarding comments in this change is illustrated here:

54be9d6#diff-a2bb675c726f8d466d83a85584878a9053dd6304d85aedf0511b8affb8074c56

With this change, that style of comment on the fields is now picked up by the parser, everything else should be the same.

I've also re-worked some of the unit tests, as I've changed some of the types exposed by this module. I've tried to make the tests a bit more flexible as whenever making a change to this module there is often quite a lot to change in the tests. Hopefully ParsedEntityDef and ParsedFieldDef can better replace LinesWithComments which is a lot more low level. I appreciate the coverage you get from the lower level tests, so I have tried to balance things here between flexibility and coverage, I've been through all the tests I had to remove and ensured that we're still covered by a new test based around the types I've added here.

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[parsonsmatt]

Oh, shoot, I'm sorry - we are about to land a PR that will port the parser to megaparsec

[danbroooks]

Ha 😅 no worries, I think that is for the best definitely.