Populate first draft of color crate (#3)

This is copied from Raph's "koloro" local repository, with lots of
fiddly changes to satisfy clippy.

color/Cargo.toml

@@ -15,7 +15,7 @@ publish = false
 [features]
 default = ["std"]
 std = []
-libm = []
+libm = ["dep:libm"]
 
 [package.metadata.docs.rs]
 all-features = true
@@ -25,5 +25,9 @@ targets = []
 
 [dependencies]
 
+[dependencies.libm]
+version = "0.2.11"
+optional = true
+
 [lints]
 workspace = true

color/src/bitset.rs

@@ -0,0 +1,50 @@
+// Copyright 2024 the Color Authors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! A simple bitset.
+
+/// A simple bitset, for representing missing components.
+#[derive(Default, Clone, Copy, PartialEq, Eq, Debug)]
+pub struct Bitset(u8);
+
+impl Bitset {
+    pub fn contains(self, ix: usize) -> bool {
+        (self.0 & (1 << ix)) != 0
+    }
+
+    pub fn set(&mut self, ix: usize) {
+        self.0 |= 1 << ix;
+    }
+
+    pub fn single(ix: usize) -> Self {
+        Self(1 << ix)
+    }
+
+    pub fn any(self) -> bool {
+        self.0 != 0
+    }
+}
+
+impl core::ops::BitAnd for Bitset {
+    type Output = Self;
+
+    fn bitand(self, rhs: Self) -> Self {
+        Self(self.0 & rhs.0)
+    }
+}
+
+impl core::ops::BitOr for Bitset {
+    type Output = Self;
+
+    fn bitor(self, rhs: Self) -> Self {
+        Self(self.0 | rhs.0)
+    }
+}
+
+impl core::ops::Not for Bitset {
+    type Output = Self;
+
+    fn not(self) -> Self::Output {
+        Self(!self.0)
+    }
+}