Pretty-print select functions

[MaximilianAlgehed]

A little bit of special casing to make nested projections pretty-print using the provided sel function instead of horrible nesting. Closes #20.

Before:

$> prettyPlan manyInconsistent
Simplified spec:
  constrained $ \ v_7 ->
    let a_5 = ProdFstW (toSimpleRep v_7) in
    let v_4 = ProdFstW (ProdSndW (toSimpleRep v_7)) in
    let v_3 = ProdFstW (ProdSndW (ProdSndW (toSimpleRep v_7))) in
    let v_2 = ProdFstW (ProdSndW (ProdSndW (ProdSndW (toSimpleRep v_7)))) in
    let v_1 =
      ProdFstW (ProdSndW (ProdSndW (ProdSndW (ProdSndW (toSimpleRep v_7))))) in
    let f_0 =
      ProdSndW (ProdSndW (ProdSndW (ProdSndW (ProdSndW (toSimpleRep v_7))))) in
    {assert $ f_0 <. a_5
     assert $ f_0 >. 10
     f_0 <- v_1
     assert $ v_1 >. v_2
     assert $ v_2 >. v_3
     assert $ v_3 >. v_4
     assert $ v_4 >. a_5
     assert $ a_5 <. 10}
SolverPlan
  a_5 :: Int <- TypeSpec [..9] [ ]
  v_4 :: Int <- assert $ v_4 >. a_5
  v_3 :: Int <- assert $ v_3 >. v_4
  v_2 :: Int <- assert $ v_2 >. v_3
  v_1 :: Int <- assert $ v_1 >. v_2
  f_0 :: Int <-
    TypeSpec [11..] [ ]
    ---
    assert $ f_0 <. a_5
  v_7 :: (Int,Int,Int,Int,Int,Int) <-
    assert $ ProdFstW (toSimpleRep v_7) ==. a_5
    assert $ ProdFstW (ProdSndW (toSimpleRep v_7)) ==. v_4
    assert $ ProdFstW (ProdSndW (ProdSndW (toSimpleRep v_7))) ==. v_3
    assert $ ProdFstW (ProdSndW (ProdSndW (ProdSndW (toSimpleRep v_7)))) ==. v_2
    assert $ ProdFstW (ProdSndW (ProdSndW (ProdSndW (ProdSndW (toSimpleRep v_7))))) ==. v_1
    assert $ ProdSndW (ProdSndW (ProdSndW (ProdSndW (ProdSndW (toSimpleRep v_7))))) ==. f_0

after:

$> prettyPlan manyInconsistent
Simplified spec:
  constrained $ \ v_7 ->
    let a_5 = sel @0 v_7 in
    let v_4 = sel @1 v_7 in
    let v_3 = sel @2 v_7 in
    let v_2 = sel @3 v_7 in
    let v_1 = sel @4 v_7 in
    let f_0 = sel @5 v_7 in
    {assert $ f_0 <. a_5
     assert $ f_0 >. 10
     f_0 <- v_1
     assert $ v_1 >. v_2
     assert $ v_2 >. v_3
     assert $ v_3 >. v_4
     assert $ v_4 >. a_5
     assert $ a_5 <. 10}
SolverPlan
  a_5 :: Int <- TypeSpec [..9] [ ]
  v_4 :: Int <- assert $ v_4 >. a_5
  v_3 :: Int <- assert $ v_3 >. v_4
  v_2 :: Int <- assert $ v_2 >. v_3
  v_1 :: Int <- assert $ v_1 >. v_2
  f_0 :: Int <-
    TypeSpec [11..] [ ]
    ---
    assert $ f_0 <. a_5
  v_7 :: (Int,Int,Int,Int,Int,Int) <-
    assert $ sel @0 v_7 ==. a_5
    assert $ sel @1 v_7 ==. v_4
    assert $ sel @2 v_7 ==. v_3
    assert $ sel @3 v_7 ==. v_2
    assert $ sel @4 v_7 ==. v_1
    assert $ sel @5 v_7 ==. f_0