`for..in` with integer bounds

[vtereshkov]

The generic for loop is very slow when simply iterating over a range of integers — see benchmarking results in #163.

Automatically analyzing the loop header for patterns like for i := 0; i < n; i++ is hard, both because it requires an infinite lookahead in the parser and because n is allowed to change during the iteration.

We could have a highly optimized for loop version similar to that used in Go since 1.22:

for i in n {   // 0 to n - 1
    foo(i)
}

or

for i in m..n {   // m to n - 1
   foo(i)
}