Speed up creating and extending packed arrays from iterators up to 63× #3225

Job	Run time
doc-lints	47s
clippy	1m 42s
godot-itest (macos)	59s
license-guard	34s
godot-itest (windows)	2m 1s
run-examples	1m 23s
rustfmt	20s
godot-itest (linux-full)	2m 23s
unit-test	2m 18s
godot-itest (linux-features-experimental)	1m 51s
godot-itest (linux-4.1)	1m 12s
godot-itest (linux-4.2)	1m 20s
godot-itest (linux-memcheck)	2m 13s
ci-status	1s
	19m 4s

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