[spec/statement.dd] Improve 'Foreach over Structs & Classes with Ranges'

[ntrel]

Link to std.range.
Add example using linked list.

Add 'Multiple Element Values' subheading.
Fix tuple -> sequence typo.
Fix tuple front example to not infinite loop & make runnable. Show single foreach variable is still legal.
Link to Tuple.

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[dkorpel]

Eventually I would like to see an example that doesn't depend on Phobos tuples, since I found it pretty hard to figure out how to get multiple foreach variables on my own types.

[ntrel]

@dkorpel Thanks for reviewing (and the other PRs). I've updated this to define Tuple in the example instead of using std.typecons.