`StackOutputs`: introduce `pop_*()` methods #4200

Job	Run time
rustfmt	19s
Clippy	1m 18s
no-std (stable, wasm32-unknown-unknown)	1m 33s
Check Rust stable on ubuntu with --all-targets --no-default-features	1m 1s
no-std (nightly, wasm32-unknown-unknown)	48s
Check Rust stable on ubuntu with --all-targets	1m 10s
Test Rust stable on ubuntu with --profile test-release	8m 58s
Check Rust stable on ubuntu with --all-targets --all-features	42s
Test Rust nightly on windows-2022	0s
Test Rust stable on ubuntu with --profile test-release --doc	5m 59s
Check Rust nightly on ubuntu with --all-targets --no-default-features	1m 29s
Test Rust nightly on ubuntu with --profile test-release	5m 8s
Check Rust nightly on ubuntu with --all-targets	1m 12s
Test Rust nightly on ubuntu with --profile test-release --doc	2m 53s
Check Rust nightly on ubuntu with --all-targets --all-features	1m 17s
	33m 47s

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