Deprecate `tree_fold1` for `tree_reduce` #816

Job	Run time
msrv	1m 1s
doc	10s
check	24s
test	49s
check (--no-default-features)	17s
check format	5s
check (--no-default-features --features use_alloc)	11s
check (--all-targets --all-features)	27s
semver-checks	2m 24s
All checks succeeded	1s
	5m 49s

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