perf: use deque for FormData.add_fields() queue

[giulio-leone]

Summary

Replace list.pop(0) with collections.deque.popleft() in FormData.add_fields().

Problem

add_fields() uses a work-list that drains from the front via .pop(0) and may grow via .extend() when MultiDict entries are encountered (BFS pattern). Each list.pop(0) is O(n) because CPython must shift all remaining elements.

Solution

Switch from list to collections.deque and use .popleft() for O(1) front removal.

Compatibility

• .extend(), truthiness, and iteration are identical on deque
• No API surface changes; purely internal optimisation
• Zero behaviour change

Complexity

Operation	Before	After
pop front	O(n)	O(1)

[codspeed-hq]

Merging this PR will not alter performance

✅ 59 untouched benchmarks

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_{Comparing giulio-leone:fix/formdata-deque (c25e610) with master (c1981ef)}