Generic assumeUnique

[Per Nordlöw]

Why isn't `std.exception.assumeUnique` generically defined as

```d
auto assumeUnique(T)(T x) @trusted {
	return cast(immutable)x;
}
```

instead of currently restricted to arrays at 
https://github.com/dlang/phobos/blob/master/std/exception.d#L905?