Improve pretty-printing of nested projections on generic types

[MaximilianAlgehed]

We have code that's pretty-printed like this in the examples:

    assert $ ProdFstW (toSimpleRep v_10) ==. as_8
    assert $ ProdFstW (ProdSndW (toSimpleRep v_10)) ==. bs_7
    assert $ ProdSndW (ProdSndW (toSimpleRep v_10)) ==. m_6

and by giving a custom prettySymbol for ProdFstW and ProdSndW that recognizes this pattern we should be able
to rewrite this code to the following during pretty-printing:

    assert $ sel @0 v_10 ==. as_8
    assert $ sel @1 v_10 ==. bs_7
    assert $ sel @2 v_10 ==. m_6

This isn't a huge win here, but it's a considerably bigger win here where the nesting is much deeper than 2 or 3.

[MaximilianAlgehed]

The logic above is obviously to look for nested ProdFst/SndW followed by toSimpleRep and count the number of ProdSnd