Rename `Reader` to `ImageReader` #1407

Job	Run time
cargo-deny	21s
Fuzz targets (afl)	2m 13s
build_benchmarks	50s
Fuzz targets (cargo-fuzz)	53s
clippy	2m 1s
public_private_dependencies	22s
rustfmt	11s
test_avif_decoding	1m 24s
test_features	27s
test_other_archs (powerpc-unknown-linux-gnu)	2m 51s
test_features (default)	1m 56s
test_toolchains (1.67.1)	34s
test_other_archs (i686-unknown-linux-gnu)	1m 46s
test_features (rayon)	40s
test_toolchains (nightly)	1m 46s
test_features (avif)	1m 15s
test_toolchains (beta)	2m 2s
test_features (bmp)	31s
test_features (dds)	37s
test_features (exr)	52s
test_features (ff)	31s
test_features (gif)	32s
test_features (hdr)	34s
test_features (ico)	53s
test_features (jpeg)	33s
test_features (png)	34s
test_features (pnm)	37s
test_features (qoi)	46s
test_features (tga)	31s
test_features (tiff)	50s
test_features (webp)	37s
	30m 30s

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