Pretty-print AST as szip, sunzip, snestS, sunNestS, etc., instead of just tconvert or worse

[Mikolaj]

This is the following TODO in the code

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

Lines 441 to 442 in 3b37bc7

	-- TODO: pretty-print correctly szip, sunzip, snestS, sunNestS
	-- or at least make sure they get printed as tconvert, not as the others

The analogous task for sfromK, etc., is already done in the code just below, but szip, etc., may be a bit more complex.

[Mikolaj]

A separate but related task is

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

Line 456 in 3b37bc7

-- TODO: this is too lax, since it accepts nests/unnests. Add tests and fix.

This code is used for term rewriting, but the required conditions on domains and codomains of terms may prove similar to the ones needed to solve the issue from the ticket description.