WIP no-std-check

- `cargo test --no-default-features --features=alloc` -> OK
- `cargo +nightly-2022-10-22 no-std-check --no-default-features` -> FAIL

Author: n-prat

Cargo.toml

@@ -31,11 +31,11 @@ path = "./src/lib.rs"
 
 [dependencies]
 bytemuck = { version = "1.7.0", features = ["extern_crate_alloc"] } # includes cast_vec
-byteorder = "1.3.2"
+byteorder = { version = "1.3.2", default-features = false }
 num-rational = { version = "0.4", default-features = false }
-num-traits = "0.2.0"
+num-traits = { version = "0.2", default-features = false }
 snafu = { version = "0.7", default-features = false }
-gif = { version = "0.12", optional = true }
+gif = { version = "0.12", optional = true, default-features = false }
 jpeg = { package = "jpeg-decoder", version = "0.3.0", default-features = false, optional = true }
 png = { version = "0.17.6", optional = true }
 scoped_threadpool = { version = "0.1", optional = true }
@@ -45,7 +45,9 @@ rgb = { version = "0.8.25", optional = true }
 mp4parse = { version = "0.12.0", optional = true }
 dav1d = { version = "0.6.0", optional = true }
 dcv-color-primitives = { version = "0.4.0", optional = true }
-color_quant = "1.1"
+# TODO(interstellar) no_std not supported on crate.io; need a fork/patch
+# NOTE: also an optional dependency of "gif"
+color_quant = { version = "1.1", git = "https://github.com/Interstellar-Network/color_quant.git", branch = "sgx-nostd-compat", default-features = false, optional = true }
 exr = { version = "1.5.0", optional = true }
 qoi = { version = "0.4", optional = true }
 libwebp = { package = "webp", version = "0.2.2", default-features = false, optional = true }
@@ -64,7 +66,7 @@ jpeg = { package = "jpeg-decoder", version = "0.3.0", default-features = false,
 # TODO: Add "avif" to this list while preparing for 0.24.0
 default = ["std", "gif", "jpeg", "ico", "png", "pnm", "tga", "tiff", "webp", "bmp", "hdr", "dxt", "dds", "farbfeld", "jpeg_rayon", "openexr", "qoi"]
 
-std = ["snafu/std"]
+std = ["snafu/std", "num-traits/std", "byteorder/std", "gif/default", "color_quant/std"]
 ico = ["bmp", "png"]
 pnm = []
 tga = []
@@ -96,6 +98,8 @@ avif-decoder = ["mp4parse", "dcv-color-primitives", "dav1d"]
 # Requires rustc nightly for feature test.
 benchmarks = []
 
+alloc = ["num-traits/libm", "color_quant/alloc"]
+
 [[bench]]
 path = "benches/decode.rs"
 name = "decode"