Rename `Reader` to `ImageReader` #1408

Job	Run time
test_features	42s
test_features (default)	1m 44s
test_features (rayon)	29s
test_features (avif)	1m 6s
test_features (bmp)	34s
Fuzz targets (afl)	1m 55s
test_features (dds)	28s
Fuzz targets (cargo-fuzz)	50s
test_features (exr)	45s
test_features (ff)	42s
test_avif_decoding	1m 34s
test_features (gif)	47s
clippy	2m 26s
test_other_archs (powerpc-unknown-linux-gnu)	2m 59s
test_features (hdr)	25s
test_other_archs (i686-unknown-linux-gnu)	1m 33s
test_toolchains (1.67.1)	36s
test_features (ico)	44s
test_toolchains (nightly)	1m 43s
test_features (jpeg)	42s
test_toolchains (beta)	1m 58s
test_features (png)	45s
build_benchmarks	52s
test_features (pnm)	42s
test_features (qoi)	41s
cargo-deny	18s
test_features (tga)	43s
public_private_dependencies	23s
test_features (tiff)	38s
test_features (webp)	34s
rustfmt	7s
	30m 25s

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