Use 'unsafe' modifier explicitly to have it visible in code snippets

[BartoszKlonowski]

This pull request fixes #42312

It removes the unsafe scope applied to the whole class code, but applies the unsafe modifier for each separate statement that requires it, so that every time the unsafe is needed, it is clearly visible included in the code snippet presented on the page.

Note: For the part with InvokeViaFunctionPointer, where I put product outside of the unsafe scope, is because the Console.WriteLine that requires that product to be in the same scope of course. But, that Console.WriteLine is not included on the page, so I'm not sure if this will not be confusing for the reader to have that product being put out of unsafe because... no reason from the reader perspective.
(Other option, that I considered was to remove the Console.WriteLine call)
Let me know, please, what is your opinion, when reviewing 🙏

[BillWagner]

This LGTM @BartoszKlonowski

I do like keeping the declaration of product outside the unsafe block. It's in keeping with the goal of keeping unsafe blocks smaller.