include generic Mainable instance for [(String, d)]

[byorgey]

Closes #293.

I left in the export of defaultMultiMainRender even though backends should have no need for it now, just to avoid a potential source of breakage.

[mention-bot]

@byorgey, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fryguybob, @cchalmers and @bergey to be potential reviewers.

[cchalmers]

LGTM. Although you might want a small comment on defaultMultiMainRender saying "Used for main bale instance of [(String, d)]" or something.

Also we'll need get rid of all the [(String, QDiagram B V2 n Any)] instances in the backends because they'll overlap. I guess this will make it a major release bump?

[byorgey]

Yes, we will need to delete those instances in the backends. Technically I suppose that would make it a major release bump, though I really doubt it will matter much.

Incidentally, the build is failing for GHC 8.2 since it can't install haddock. I'm not sure what to do about that.