Speed up toLower

[Bodigrim]

Fixes #459. CC @noughtmare

[phadej]

What about toCaseFold, toUpper and toTitle?

[Bodigrim]

Should be improved in the same way, this is just a quick draft.

[Bodigrim]

Benchmarks:

tiny
  toLower
    Text:     OK (1.46s)
      42.6 ns ± 3.8 ns, 74% faster than baseline
    LazyText: OK (3.70s)
      54.6 ns ± 1.6 ns, 74% faster than baseline
  toUpper
    Text:     OK (5.90s)
      43.5 ns ± 2.5 ns, 75% faster than baseline
    LazyText: OK (1.90s)
      56.1 ns ± 4.5 ns, 75% faster than baseline
ascii-small
  toLower
    Text:     OK (6.97s)
      212  μs ±  12 μs, 88% faster than baseline
    LazyText: OK (1.82s)
      216  μs ±  17 μs, 89% faster than baseline
  toUpper
    Text:     OK (2.13s)
      258  μs ±  25 μs, 89% faster than baseline
    LazyText: OK (4.40s)
      264  μs ±  10 μs, 90% faster than baseline
ascii
  toLower
    Text:     OK (1.69s)
      182  ms ±  17 ms, 90% faster than baseline
    LazyText: OK (15.15s)
      233  ms ± 6.9 ms, 87% faster than baseline
  toUpper
    Text:     OK (3.79s)
      220  ms ±  17 ms, 90% faster than baseline
    LazyText: OK (4.33s)
      264  ms ±  21 ms, 89% faster than baseline
english
  toLower
    Text:     OK (6.17s)
      11.7 ms ± 227 μs, 90% faster than baseline
    LazyText: OK (3.13s)
      12.0 ms ± 537 μs, 90% faster than baseline
  toUpper
    Text:     OK (2.05s)
      15.7 ms ± 1.4 ms, 89% faster than baseline
    LazyText: OK (8.14s)
      15.5 ms ± 690 μs, 90% faster than baseline
russian
  toLower
    Text:     OK (2.22s)
      67.9 μs ± 6.3 μs, 66% faster than baseline
    LazyText: OK (2.13s)
      62.9 μs ± 4.0 μs, 71% faster than baseline
  toUpper
    Text:     OK (2.22s)
      136  μs ±  13 μs, 44% faster than baseline
    LazyText: OK (2.24s)
      135  μs ± 6.9 μs, 50% faster than baseline
japanese
  toLower
    Text:     OK (2.90s)
      43.2 μs ± 2.5 μs, 66% faster than baseline
    LazyText: OK (3.01s)
      44.7 μs ± 3.0 μs, 67% faster than baseline
  toUpper
    Text:     OK (1.49s)
      44.1 μs ± 4.0 μs, 64% faster than baseline
    LazyText: OK (2.96s)
      45.1 μs ± 2.4 μs, 66% faster than baseline

[noughtmare]

I can confirm that this also makes the Haskell code almost as fast as Python's standard library:

Benchmark 1: haskell/simple-exe <kjvbible.txt
  Time (mean ± σ):      80.4 ms ±   3.2 ms    [User: 64.2 ms, System: 7.0 ms]
  Range (min … max):    71.7 ms …  89.2 ms    35 runs

Benchmark 1: python3 simple.py <kjvbible.txt
  Time (mean ± σ):      77.0 ms ±   1.1 ms    [User: 69.1 ms, System: 5.3 ms]
  Range (min … max):    75.7 ms …  81.5 ms    37 runs

[Lysxia]

LGTM.

Interesting trend that streaming is progressively losing ground.