[stdlib] Implement difference between pointers

[msaelices]

• [stdlib] Implement difference between pointers, so we have completed the pointers arithmetic we can do in C
• [docs] Document pointer arithmetic in pointers document

To complete the pointers arithmetic that we can do it in C

❯ mojo
  1> s = 100 
  2.  
(Int) s = 100
  2> p = UnsafePointer.address_of(s) 
  3.  
(UnsafePointer[Int, 0, 0, alignof[::AnyType,__mlir_type.!kgen.target]() if triple_is_nvidia_cuda() else 1]) p = {
  (pointer<_stdlib::_builtin::_int::_Int>) address = 0x000071858be84008
}
  3> p2 = p + 1 
  4.  
(UnsafePointer[Int, 0, 0, alignof[::AnyType,__mlir_type.!kgen.target]() if triple_is_nvidia_cuda() else 1]) p2 = {
  (pointer<_stdlib::_builtin::_int::_Int>) address = 0x000071858be84010
}
  4> offset = p2 - p 
  5.  
(Int) offset = 1  

[JoeLoser]

Can you add a unit test, please?

[msaelices]

Can you add a unit test, please?

Sure. Done: msaelices@19830ec

[JoeLoser]

!sync