review: refactor(SpoonifierVisitor): Deque should be used instead of Stack

[tenax66]

Unless there's particular reason, Deque should be used instead of Stack (like other part of spoon code).

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[tenax66]

After class replacement, several tests failed due to peek() of Deque may return null instead of throwing an Exception.
So I replaced peek() with getFirst(), which is said to be equivalent to peek() of Stack in Javadoc of Deque.

[SirYwell]

Looks good, will merge.

[SirYwell]

Thanks!