Drop MemoryLayout::map

The MemoryLayout::map method is an attempt at defining transformation on layouts. Unfortunately, this way of transforming layouts is very limited, as it only allows to replace one layout with another in a location at a time.

As such, it is not possible, for instance, to define a global layout transformation which e.g. flips the endianness of all value layouts nested in a given layout.

It would be better to omit this method from the API, as it takes a good name (MemoryLayout::map) without providing the necessary flexibility.