Rename Option to MyOption in tutorial to workaround iteration bug.

 Fix for #15409. 

I opted to rename `Option` to `MyOption` so we won't get an error if we simply type in all code. 

Explicitly mention that rust has an `Option` type and in the following code snippets we use rust's built in type. It should be clear what's going on I hope.
Also, maybe it should not be 'constants', but 'variants'