Add `rayon` parallel iterators #1067

Job	Run time
cargo-deny	19s
build_benchmarks	45s
Fuzz targets (afl)	2m 18s
Fuzz targets (cargo-fuzz)	4m 12s
clippy	2m 16s
public_private_dependencies	19s
rustfmt	13s
test_avif	1m 17s
test_avif_decoding	1m 22s
test_big_endian	40s
test_toolchains (1.61.0)	31s
test_toolchains (nightly)	1m 15s
test_big_endian (default)	2m 15s
test_features	27s
test_big_endian (webp)	1m 17s
test_toolchains (beta)	1m 11s
test_features (default)	1m 5s
test_big_endian (webp-encoder)	1m 58s
test_features (gif)	25s
verify_msrv	3m 52s
test_features (jpeg)	30s
test_features (png)	35s
test_features (tiff)	34s
test_features (ico)	34s
test_features (pnm)	24s
test_features (tga)	21s
test_features (webp)	26s
test_features (bmp)	30s
test_features (hdr)	28s
test_features (dxt)	22s
test_features (dds)	26s
test_features (farbfeld)	24s
test_features (openexr)	36s
test_features (jpeg_rayon)	30s
test_features (webp-encoder)	53s
	35m 30s

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