This PR implements RFC #562 , it was given a green light long ago.

The code is based on beef's idea and impl in slicerator/itermacros.

For convenient syntax that allows writing "foo".split.mapIt(it).collect() and not mapIt("foo".split, it).collect()" it requires merging #25679 (or an alternative fix) but it doesn't rely on it inherently (except some examples and test cases assume this form being available), so workable even without it.

This example works now:

import itertools
import std/[strutils, options]

# Find the first element in a sequence of transformed numbers above 35.
# Note using `Slice[int].items` instead of `CountUp` (also supported).
doAssert (-25..25).items.mapIt(it * 10 div 7).findIt(it > 35) == none(int)

# Filter philosophers by country, compose sentences, join to a string.
let philosophers = {
  "Plato": "Greece", "Aristotle": "Greece", "Socrates": "Greece",
  "Confucius": "China", "Descartes": "France"}

const Phrase = "$1 is a famous philosopher from $2."
let facts = philosophers.items()
              .filterIt(it[1] != "Greece")
              .mapIt([it[0], it[1]])
              .mapIt(Phrase % it)
              .foldIt("", acc & it & '\n')
doAssert facts == """
Confucius is a famous philosopher from China.
Descartes is a famous philosopher from France.
"""

The implementation is straightforward, API is bog-standard and shouldn't be surprising to anyone familiar with this style, tests cover everything. The only interesting thing in the code is that it uses concepts for overloads without any when compiles hacks, feels pretty great.

The set of provided templates is basic and the naming is all common, so I hope there's not much place for bikeshedding. I've been craving for this feature since 2018 or so and didn't hear much pushback against the idea, so I hope this PR doesn't go to waste.

requires #25679