class LayoutWithStaticSizes

`Layout` knows the element types of the arrays we want to lay out in

memory but not the number of elements in each array.

`Partial(size1, ..., sizeN)` allows us to specify the latter. The

resulting immutable object can be used to obtain pointers to the

individual arrays.

It's allowed to pass fewer array sizes than the number of arrays. E.g.,

if all you need is to the offset of the second array, you only need to

pass one argument -- the number of elements in the first array.

// int[3] followed by 4 bytes of padding and an unknown number of

// doubles.

auto x = Layout

<int

, double>::Partial(3);

// doubles start at byte 16.

assert(x.Offset

<

1>() == 16);

If you know the number of elements in all arrays, you can still call

`Partial()` but it's more convenient to use the constructor of `Layout`.

Layout

<int

, double> x(3, 5);

Note: The sizes of the arrays must be specified in number of elements,

not in bytes.

Requires: `sizeof...(Sizes) + NumStaticSizes

<

= sizeof...(Ts)`.

Requires: all arguments are convertible to `size_t`.