Use inout for ref returning members in std.container.array

For motivation see 

http://forum.dlang.org/thread/agyeejbrpotpjrecepyl@forum.dlang.org#post-wysehvbsfnezfozsagpl:40forum.dlang.org
I don't think this works when `T` in `Array!T` has mutable indirection. Unfortunately, constifying `opSlice` is not a satisfying solution to this problem.