Implement foldl-related folds for PostOrder

[meooow25]

PostOrder is able to incrementally produce elements from the end of
sequence represented by the Foldable instance.

Closes #1247

---

Benchmarks with GHC 9.14:

Name                                                           Time - - - - - - - -    Allocated - - - - -
                                                                    A       B     %         A       B     %
PostOrder.Foldable.foldl1.bin,n=1000                           5.2 μs  2.4 μs  -54%     77 KB   31 KB  -59%
PostOrder.Foldable.foldl1.bin,n=1000000                        8.0 ms  2.9 ms  -63%     76 MB   31 MB  -59%
PostOrder.Foldable.foldl1.line,n=1000                          2.0 μs  2.0 μs   +2%     23 KB   23 KB   +0%
PostOrder.Foldable.foldl1.line,n=1000000                       1.9 ms  2.0 ms   +3%     23 MB   23 MB   +0%
PostOrder.Foldable.folds.bin,n=1000.foldl_cpsOneShotSum         13 μs  5.0 μs  -61%    164 KB   70 KB  -57%
PostOrder.Foldable.folds.bin,n=1000.foldl_cpsSum                13 μs  9.2 μs  -29%    164 KB  133 KB  -19%
PostOrder.Foldable.folds.bin,n=1000.foldl_elem                 5.3 μs  2.4 μs  -55%     78 KB   31 KB  -60%
PostOrder.Foldable.folds.bin,n=1000.foldl_traverseSum           12 μs  7.1 μs  -41%    133 KB  101 KB  -23%
PostOrder.Foldable.folds.bin,n=1000.foldr'_maximum              13 μs  2.9 μs  -77%    149 KB  160 B   -99%
PostOrder.Foldable.folds.bin,n=1000.foldr'_sum                  13 μs  2.1 μs  -83%    164 KB   16 B   -99%
PostOrder.Foldable.folds.bin,n=1000000.foldl_cpsOneShotSum      15 ms  5.9 ms  -60%    160 MB   69 MB  -57%
PostOrder.Foldable.folds.bin,n=1000000.foldl_cpsSum             14 ms   11 ms  -21%    160 MB  130 MB  -19%
PostOrder.Foldable.folds.bin,n=1000000.foldl_elem              8.2 ms  3.1 ms  -62%     76 MB   31 MB  -60%
PostOrder.Foldable.folds.bin,n=1000000.foldl_traverseSum        13 ms  8.2 ms  -39%    130 MB   99 MB  -23%
PostOrder.Foldable.folds.bin,n=1000000.foldr'_maximum           14 ms  3.5 ms  -75%    145 MB  624 B   -99%
PostOrder.Foldable.folds.bin,n=1000000.foldr'_sum               14 ms  2.8 ms  -80%    160 MB  316 B   -99%
PostOrder.Foldable.folds.line,n=1000.foldl_cpsOneShotSum        11 μs  4.0 μs  -63%    164 KB   70 KB  -57%
PostOrder.Foldable.folds.line,n=1000.foldl_cpsSum               11 μs  7.4 μs  -32%    164 KB  133 KB  -19%
PostOrder.Foldable.folds.line,n=1000.foldl_elem                4.7 μs  2.0 μs  -57%     78 KB   31 KB  -60%
PostOrder.Foldable.folds.line,n=1000.foldl_traverseSum         9.8 μs  6.1 μs  -37%    133 KB  101 KB  -23%
PostOrder.Foldable.folds.line,n=1000.foldr'_maximum             12 μs  2.6 μs  -77%    164 KB   16 KB  -90%
PostOrder.Foldable.folds.line,n=1000.foldr'_sum                 11 μs  2.0 μs  -81%    164 KB   16 B   -99%
PostOrder.Foldable.folds.line,n=1000000.foldl_cpsOneShotSum     11 ms  4.1 ms  -63%    160 MB   69 MB  -57%
PostOrder.Foldable.folds.line,n=1000000.foldl_cpsSum            11 ms  7.1 ms  -37%    160 MB  130 MB  -19%
PostOrder.Foldable.folds.line,n=1000000.foldl_elem             6.6 ms  1.9 ms  -71%     76 MB   31 MB  -60%
PostOrder.Foldable.folds.line,n=1000000.foldl_traverseSum      9.8 ms  6.1 ms  -38%    130 MB   99 MB  -23%
PostOrder.Foldable.folds.line,n=1000000.foldr'_maximum          12 ms  2.6 ms  -78%    160 MB   15 MB  -90%
PostOrder.Foldable.folds.line,n=1000000.foldr'_sum              11 ms  2.0 ms  -82%    160 MB  649 B   -99%
PostOrder.Foldable1.foldlMap1.bin,n=1000                       3.1 μs  3.1 μs   +0%     62 KB   62 KB   +0%
PostOrder.Foldable1.foldlMap1.bin,n=1000000                    3.7 ms  3.9 ms   +4%     61 MB   61 MB   +0%
PostOrder.Foldable1.foldlMap1.line,n=1000                      3.4 μs  3.3 μs   -3%     62 KB   62 KB   +0%
PostOrder.Foldable1.foldlMap1.line,n=1000000                    29 ms   30 ms   +1%     61 MB   61 MB   +0%