walkBack: use Unqual & foreach vs. checking empty twice

result

> ldc -O5 -release -boundscheck=off walkBack.d && ./walkBack

walkBack.foreach = 3 secs, 772 ms, 463 μs, and 6 hnsecs
walkBack.while  = 4 secs, 191 ms, 417 μs, and 8 hnsecs

walkBack.d

import std.algorithm, std.conv, std.datetime, std.functional, std.range, std.stdio, std.traits, std.typecons;

private void doNotOptimizeAway(T)(auto ref T t)
{
    import core.thread : getpid;
    import std.stdio : writeln;
    if(getpid() == 1) {
        writeln(*cast(char*)&t);
    }
    assert(t == 99_999);
}

void main()
{
    auto n = 100_000;
    auto range = n.iota!size_t.array;
    alias Range = typeof(range);

    writeln("starting benchmark");
    auto bench = benchmark!(
        { doNotOptimizeAway({
            Unqual!(ElementType!Range) last;
            foreach (e; range)
              last = cast(Unqual!(ElementType!Range)) e;
            return last;
        }());},
        { doNotOptimizeAway({
            while(!range.empty){
                auto element = range.front;
                range.popFront();
                if(range.empty) return element;
            }
            return range.front;
        }());},
    )(1_000_000_000);

    string[] names = ["walkBack.foreach", "walkBack.while"];
    foreach(j,r;bench)
        writefln("%-15s = %s", names[j], r.to!Duration);
}