Rename `Reader` to `ImageReader` #1409

Job	Run time
cargo-deny	20s
build_benchmarks	48s
Fuzz targets (afl)	2m 36s
Fuzz targets (cargo-fuzz)	56s
clippy	1m 59s
public_private_dependencies	25s
rustfmt	6s
test_avif_decoding	1m 26s
test_features	46s
test_other_archs (powerpc-unknown-linux-gnu)	2m 42s
test_features (default)	1m 51s
test_toolchains (1.67.1)	32s
test_other_archs (i686-unknown-linux-gnu)	1m 41s
test_features (rayon)	40s
test_toolchains (nightly)	1m 10s
test_features (avif)	1m 3s
test_toolchains (beta)	1m 20s
test_features (bmp)	36s
test_features (dds)	48s
test_features (exr)	46s
test_features (ff)	41s
test_features (gif)	28s
test_features (hdr)	37s
test_features (ico)	49s
test_features (jpeg)	31s
test_features (png)	38s
test_features (pnm)	34s
test_features (qoi)	34s
test_features (tga)	32s
test_features (tiff)	36s
test_features (webp)	51s
	29m 22s

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