Add A.flatMap

[ivan-kleshnin]

This crucial function is missing. I guess, accidentally because other namespaces expose it.
For example: https://mobily.github.io/ts-belt/api/option#flatmap
I've double checked it's not exposed under a different name like chain or concatMap.

[mobily]

@ivan-kleshnin added in v4.0.0-rc.5 🚀 14a5c15

benchmarks:

flatMap (single function call)

✔  @mobily/ts-belt  27,383,074.99  ops/sec  ±0.33%  (99 runs)  fastest
██████████████████████████████████████████████████████████████████████


✔  remeda            1,759,670.66  ops/sec  ±1.36%  (97 runs)  -93.57%
████


✔  ramda             1,392,700.93  ops/sec  ±0.52%  (91 runs)  -94.91%
███


✔  rambda            4,870,498.47  ops/sec  ±0.92%  (98 runs)  -82.21%
████████████


✔  lodash/fp         5,749,906.26  ops/sec  ±0.78%  (87 runs)  -79.00%
██████████████

→ Fastest is @mobily/ts-belt

flatMap (function call inside pipe)

✔  @mobily/ts-belt  21,116,789.82  ops/sec  ±2.48%  (94 runs)  fastest
██████████████████████████████████████████████████████████████████████


✔  remeda            2,500,686.11  ops/sec  ±1.44%  (98 runs)  -88.16%
████████


✔  ramda               872,490.77  ops/sec  ±0.77%  (92 runs)  -95.87%
██


✔  rambda            4,248,478.35  ops/sec  ±0.54%  (93 runs)  -79.88%
██████████████


✔  lodash/fp           896,410.20  ops/sec  ±1.63%  (93 runs)  -95.75%
██

→ Fastest is @mobily/ts-belt