Simplify cargo features exposed by the crate #913

Job	Run time
build_benchmarks	1m 9s
cargo-deny	32s
Fuzz targets (afl)	1m 47s
Fuzz targets (cargo-fuzz)	4m 29s
clippy	2m 32s
public_private_dependencies	47s
rustfmt	16s
test_avif	1m 19s
test_avif_decoding	2m 3s
test_big_endian	55s
test_big_endian (default)	1m 14s
test_big_endian (webp-native)	53s
test_features	21s
test_features (default)	1m 15s
test_features (gif)	25s
test_features (jpeg)	21s
test_features (png)	21s
test_features (tiff)	20s
test_features (ico)	33s
test_features (pnm)	23s
test_features (tga)	25s
test_features (webp)	22s
test_features (bmp)	23s
test_features (hdr)	33s
test_features (dxt)	28s
test_features (dds)	28s
test_features (ff)	25s
test_features (exr)	32s
test_features (parallel)	23s
test_features (webp-native)	32s
test_toolchains (1.64.0)	1m 37s
test_toolchains (nightly)	1m 7s
test_toolchains (beta)	1m 32s
verify_msrv	2m 41s
	33m 23s

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