Tutorial: make struct section more coherent

In struct section of tutorial, make everything more coherent and
clear by always using "struct Point".
Also, do not prematurely introduce pointers and arrays.
Fixes #5240

Signed-off-by: Luca Bruno <lucab@debian.org>
auto merge of #5441 : lucab/rust/lucab/tutorial/struct, r=pcwalton

In struct section of tutorial, make everything more coherent and
clear by always using "struct Point".
Also, do not prematurely introduce pointers and arrays.
Fixes #5240