Add `map_err` and `err_into` iterator adapters #340

Job	Run time
test	1m 18s
check (msrv)	1m 34s
check (msrv, --no-default-features)	1m 38s
check (msrv, --no-default-features --features use_alloc)	1m 48s
check (stable)	16s
check (stable, --no-default-features)	15s
check (stable, --no-default-features --features use_alloc)	17s
check (stable, --all-targets --all-features)	28s
bors build finished	1s
	7m 35s

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