Add `Of<T>` alignment

[V0ldek]

Is your feature request related to a problem? Please describe.
It might be useful to have a type that aligns to the alignment required for a given type.

1. Defining PointerSize (Add PointerSize alignment #6) would become an alias for Of<usize>.
2. When generic SIMD gets stabilised, this could be used to define SIMD alignment for free.

Describe the solution you'd like
Define the alignment::Of<T> type that will provide Alignment impl returning std::mem::align_of::<T>() as its size. This is safe, since a proper alignment of T is necessarily a proper alignment in general.