Add a number of methods to `EitherOrBoth` #345

Job	Run time
test	1m 30s
check (msrv)	1m 14s
check (msrv, --no-default-features)	56s
check (msrv, --no-default-features --features use_alloc)	1m 10s
check (stable)	8s
check (stable, --no-default-features)	8s
check (stable, --no-default-features --features use_alloc)	17s
check (stable, --all-targets --all-features)	27s
bors build finished	1s
	5m 51s

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