I don't know where such a tool should finally be placed in D, but I having it
available as a library or as part of the language would be great. It seems like a
lot of other languages have it like python, perl, C++, and Java. So it can't be
that useless.

There is fill() in std.algorithm, but it needs an already present range...

auto s = new int[5];

fill(s, 10);

s is now [10,10,10,10,10]