Speed up creating and extending packed arrays from iterators up to 63× #3298

Job	Run time
unit-test	2m 22s
rustfmt	18s
clippy	1m 53s
doc-lints	45s
license-guard	31s
godot-itest (macos)	1m 20s
run-examples	1m 20s
godot-itest (windows)	2m 24s
godot-itest (linux-full)	2m 32s
godot-itest (linux-features-experimental)	2m 16s
godot-itest (linux-4.1)	1m 7s
godot-itest (linux-4.2)	1m 20s
godot-itest (linux-memcheck)	2m 18s
ci-status	3s
	20m 29s

Provide feedback