Add `PointerSize` alignment

[V0ldek]

Is your feature request related to a problem? Please describe.
Allow aligning to the target's pointer size.

Describe the solution you'd like
There should be an alignment::PointerSize type that gives an alignment equal to the size of usize.

Describe alternatives you've considered
This can be defined manually using TwoTo<N> and a const function calculating the logarithm of std::mem::size_of::<usize>(), but that's tedious.

[V0ldek]

This would be immediately solved by #7