std-algorithm-iteration-each-early-stopping.dd

`std.algorithm.iteration.each` is now capable of early-stopping

$(REF each, std,algorithm,iteration) is now capable of exiting early.
When a `No.each` $(REF Flag, std,typecons) is returned from the function that
is called by `each`, the iteration will be aborted early.
Analogously, returning `Yes.each` will continue the iteration.
For example:

---
auto arr = [10, 20, 30];
arr.each!((n) { arr ~= n; return (n == 20) ? No.each : Yes.each; }); // aborts after the second iteration
assert(arr == [10, 20, 30, 10, 20]);
---