Isolate unsafe.Sizeof in a wrapper in a dedicated file

[ccoVeille]

unsafe.Sizeof is used in various place here in this file

Please consider:

• creating a sizeof.go file

• add a sizeOfNumber func

• add this exact comment

     // NOTE: [unsafe.SizeOf] simply returns the size in bytes of the value. 
     // For primitive types T, the generic stencil is precompiled and this value 
     // is resolved at compile time
  

• call sizeOfNumber everywhere else

This way, only sizeof.go will import the unsafe package and it would be clearer

Originally posted by @ccoVeille in #108 (comment)

ok thanks. I'v been considering that wrapper func too. Perhaps in a follow-up. Now it is time to merge and move on to the next series of packages

Originally posted by @fredbi in #108 (comment)