Marcin Szamotulski authored Mar 16, 2021 and David Feuer committed Jun 28, 2021

* Map.foldr'
* Map.foldl'
* Map.foldrWithKey'
* Map.foldlWithKey'
* Set.foldr'
* Set.foldl'

They now evaluate intermediate results of the accumulator before
appling the folding operator again.

This patch contains tests based on `nothunks` library.

Benchmark summary:

Map old:
```
benchmarking foldlWithKey' ...
benchmarked foldlWithKey'
time                 28.57 μs   (28.00 μs .. 29.35 μs)
                     0.991 R²   (0.979 R² .. 0.999 R²)
mean                 28.43 μs   (28.12 μs .. 28.99 μs)
std dev              1.358 μs   (729.0 ns .. 2.101 μs)
variance introduced by outliers: 27% (moderately inflated)

benchmarking foldrWithKey' ...
benchmarked foldrWithKey'
time                 80.25 ns   (79.45 ns .. 81.13 ns)
                     0.998 R²   (0.996 R² .. 0.999 R²)
mean                 79.84 ns   (79.27 ns .. 80.68 ns)
std dev              2.184 ns   (1.572 ns .. 2.921 ns)
variance introduced by outliers: 11% (moderately inflated)
```

Map new:
```
benchmarking foldlWithKey' ...
benchmarked foldlWithKey'
time                 27.76 μs   (27.15 μs .. 28.34 μs)
                     0.996 R²   (0.992 R² .. 0.998 R²)
mean                 27.08 μs   (26.84 μs .. 27.43 μs)
std dev              964.2 ns   (720.7 ns .. 1.350 μs)
variance introduced by outliers: 18% (moderately inflated)

benchmarking foldrWithKey' ...
benchmarked foldrWithKey'
time                 74.02 ns   (73.01 ns .. 75.82 ns)
                     0.998 R²   (0.994 R² .. 1.000 R²)
mean                 73.14 ns   (72.91 ns .. 73.80 ns)
std dev              1.245 ns   (434.3 ps .. 2.625 ns)
```

Set old:

```benchmarking member ...
benchmarked member
time                 237.1 μs   (231.5 μs .. 246.0 μs)
                     0.993 R²   (0.986 R² .. 0.999 R²)
mean                 234.3 μs   (232.1 μs .. 238.0 μs)
std dev              9.031 μs   (5.737 μs .. 15.46 μs)
variance introduced by outliers: 20% (moderately inflated)
```

Set new:
```
benchmarking member ...
benchmarked member
time                 219.4 μs   (216.3 μs .. 222.6 μs)
                     0.999 R²   (0.997 R² .. 1.000 R²)
mean                 221.3 μs   (219.3 μs .. 225.3 μs)
std dev              10.25 μs   (5.408 μs .. 18.71 μs)
variance introduced by outliers: 28% (moderately inflated)
```