Hoist general_subtree() helper out of loops #601

Job	Run time
Format	4s
Clippy	17s
Cargo Deny	10s
Check for documentation errors (stable)	2m 45s
Check for documentation errors (beta)	33s
Check for documentation errors (nightly)	24s
Build+test (--all-features, stable)	19s
Build+test (stable, ubuntu-20.04)	26s
Build+test (--features=alloc, stable, ubuntu-20.04)	27s
Build+test (--no-default-features, stable, ubuntu-20.04)	21s
Build+test (--all-features, --release, stable, ubuntu-20.04)	1m 36s
Build+test (--all-features, nightly, ubuntu-20.04)	34s
Build+test (--all-features, beta, ubuntu-20.04)	30s
Check cross compilation targets	43s
MSRV	1m 39s
Cargo Package	11s
Check semver compatibility	3m 52s
Measure coverage	43s
	15m 34s

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