Get pretty-printed AST closer to round-tripping

[Mikolaj]

This is the following TODO in the code

horde-ad/src/HordeAd/Core/AstPrettyPrint.hs

Lines 31 to 33 in 3b37bc7

	-- TODO: ensure that terms roundtrip if neither loseRoudtrip
	-- nor ignoreNestedLambdas is set and that explicit sharing is then preserved
	-- as opposed to displaying sharing as Haskell lets.

The goal is to make the output of pretty-printing accepted by Haskell and without losing the explicit sharing the AST provides. First we'd need small tests of the round-trip property, which will involved invoking GHC on the strings produced by pretty-printing. Then we can make more and more tests pass, so this issue has many subtasks (and so may be fixed gradually, by many PRs). The changes are bound to break some existing tests, so the failing old test results need to be fixed every time pretty-printing is modified.