dup and arrays

[Ellery Newcomer]

I just noticed that dup does not dup deep.

In a two second search I couldn't find any reason for or against, but 
I'd kinda like it if

auto r2 = r.dup;
r2[i][j] = 0;
r[i][j] = 1;
assert(r2[i][j] != r[i][j]);

held.